gh-36440: trying ruff on some posets files
    
This is changing two files in the posets folder.

- use a smaller doctest in one place
- replace one annoying use of `deepcopy` by the method `copy` of
`DiGraph`
- apply `ruff check --fix --select UP src/sage/combinat/posets` to make
a few automatic modifications, about strings mostly, and about the
syntax of `super`
- avoid one symbolic number in one doctest

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: #36440
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/combinat/posets/incidence_algebras.py

@@ -58,7 +58,7 @@ def __init__(self, R, P, prefix='I'):
 
         TESTS::
 
-            sage: P = posets.BooleanLattice(4)
+            sage: P = posets.BooleanLattice(3)
             sage: I = P.incidence_algebra(QQ)
             sage: TestSuite(I).run()  # long time
         """
@@ -93,8 +93,7 @@ def _repr_(self):
             Incidence algebra of Finite lattice containing 16 elements
              over Rational Field
         """
-        return "Incidence algebra of {} over {}".format(self._poset,
-                                                        self.base_ring())
+        return f"Incidence algebra of {self._poset} over {self.base_ring()}"
 
     def _coerce_map_from_(self, R):
         """

src/sage/combinat/posets/posets.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # sage.doctest: needs sage.graphs sage.modules
 r"""
 Finite posets
@@ -287,9 +286,8 @@
 # ****************************************************************************
 from __future__ import annotations
 from collections import defaultdict
-from copy import copy, deepcopy
+from copy import copy
 from itertools import product
-from typing import List
 
 from sage.misc.cachefunc import cached_method
 from sage.misc.lazy_attribute import lazy_attribute
@@ -706,7 +704,7 @@ def Poset(data=None, element_labels=None, cover_relations=False, linear_extensio
     if data is None:  # type 0
         D = DiGraph()
     elif isinstance(data, DiGraph):  # type 4
-        D = deepcopy(data)
+        D = data.copy(immutable=True)
     elif isinstance(data, dict):  # type 3: dictionary of upper covers
         D = DiGraph(data, format="dict_of_lists")
     elif isinstance(data, (list, tuple)):  # types 1, 2, 3 (list/tuple)
@@ -1319,7 +1317,7 @@ def __call__(self, element):
         """
         if self._is_facade and element in self._element_to_vertex_dict:
             return element
-        return super(FinitePoset, self).__call__(element)
+        return super().__call__(element)
 
     def hasse_diagram(self):
         r"""
@@ -1403,7 +1401,7 @@ def _repr_(self):
             sage: M._repr_()
             'Finite meet-semilattice containing 3 elements'
         """
-        s = "%s containing %s elements" % (self._desc, self._hasse_diagram.order())
+        s = f"{self._desc} containing {self._hasse_diagram.order()} elements"
         if self._with_linear_extension:
             s += " with distinguished linear extension"
         return s
@@ -1444,13 +1442,13 @@ def _rich_repr_(self, display_manager, **kwds):
                 return output
         # create text for non-graphical output
         if can_plot:
-            text = '{0} (use the .plot() method to plot)'.format(repr(self))
+            text = f'{repr(self)} (use the .plot() method to plot)'
         else:
             text = repr(self)
         # latex() produces huge tikz environment, override
         tp = display_manager.types
         if (prefs.text == 'latex' and tp.OutputLatex in display_manager.supported_output()):
-            return tp.OutputLatex(r'\text{{{0}}}'.format(text))
+            return tp.OutputLatex(fr'\text{{{text}}}')
         return tp.OutputPlainText(text)
 
     def __iter__(self):
@@ -2669,7 +2667,7 @@ def relations_number(self):
     # Maybe this should also be deprecated.
     intervals_number = relations_number
 
-    def linear_intervals_count(self) -> List[int]:
+    def linear_intervals_count(self) -> list[int]:
         """
         Return the enumeration of linear intervals w.r.t. their cardinality.
 
@@ -2799,10 +2797,10 @@ def is_incomparable_chain_free(self, m, n=None) -> bool:
             sage: Q = Poset({0:[2], 1:[2], 2:[3], 3:[4], 4:[]})
             sage: Q.is_incomparable_chain_free(2, 20/10)
             True
-            sage: Q.is_incomparable_chain_free(2, pi)                                   # needs sage.symbolic
+            sage: Q.is_incomparable_chain_free(2, 1.5)
             Traceback (most recent call last):
             ...
-            TypeError: 2 and pi must be integers
+            TypeError: 2 and 1.5... must be integers
             sage: Q.is_incomparable_chain_free(2, -1)
             Traceback (most recent call last):
             ...
@@ -2850,9 +2848,9 @@ def is_incomparable_chain_free(self, m, n=None) -> bool:
             try:
                 m, n = Integer(m), Integer(n)
             except TypeError:
-                raise TypeError("%s and %s must be integers" % (m, n))
+                raise TypeError(f"{m} and {n} must be integers")
             if m < 1 or n < 1:
-                raise ValueError("%s and %s must be positive integers" % (m, n))
+                raise ValueError(f"{m} and {n} must be positive integers")
             twochains = digraphs.TransitiveTournament(m) + digraphs.TransitiveTournament(n)
             if closure.subgraph_search(twochains, induced=True) is not None:
                 return False
@@ -6348,7 +6346,7 @@ def graphviz_string(self, graph_string="graph", edge_string="--"):
             s += '"%s";' % v
         s += '\n'
         for u, v in self.cover_relations_iterator():
-            s += '"%s"%s"%s";' % (v, edge_string, u)
+            s += f'"{v}"{edge_string}"{u}";'
         s += "\n}"
         return s
 
@@ -8152,9 +8150,9 @@ def is_eulerian(self, k=None, certificate=False):
             try:
                 k = Integer(k)
             except TypeError:
-                raise TypeError("parameter 'k' must be an integer, not {0}".format(k))
+                raise TypeError(f"parameter 'k' must be an integer, not {k}")
             if k <= 0:
-                raise ValueError("parameter 'k' must be positive, not {0}".format(k))
+                raise ValueError(f"parameter 'k' must be positive, not {k}")
 
         if not self.is_bounded():
             raise ValueError("the poset is not bounded")
@@ -8880,7 +8878,7 @@ def _macaulay2_init_(self, macaulay2=None):
         H = self._hasse_diagram
         txt = 'needsPackage "Posets";'
         txt += "poset({%s},{" % ','.join(str(x) for x in H)
-        txt += ",".join("{%s,%s}" % (str(x), str(y))
+        txt += ",".join(f"{{{str(x)},{str(y)}}}"
                         for x, y in H.cover_relations_iterator())
         return txt + "})"
 
@@ -8993,8 +8991,7 @@ def cardinality(self, from_iterator=False):
                         68275077901156, 4483130665195087]
         if not from_iterator and self._n < len(known_values):
             return Integer(known_values[self._n])
-        else:
-            return super(FinitePosets_n, self).cardinality()
+        return super().cardinality()
 
 
 # For backward compatibility of pickles of the former Posets()