gh-36413: various details in schemes
    
This is fixing a few details as suggested by ruff.

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: #36413
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/schemes/affine/affine_subscheme.py

@@ -484,7 +484,7 @@ def multiplicity(self, P):
             sage: X.multiplicity(Q)                                                     # needs sage.libs.singular
             1
         """
-        if not self.base_ring() in Fields():
+        if self.base_ring() not in Fields():
             raise TypeError("subscheme must be defined over a field")
 
         # check whether P is a point on this subscheme

src/sage/schemes/curves/closed_point.py

@@ -418,7 +418,7 @@ def affine(self, i=None):
         ideal = self.prime_ideal()
         if i is None:
             for j in range(P.ngens()):
-                if not P.gen(j) in ideal:
+                if P.gen(j) not in ideal:
                     i = j
                     break
         else:

src/sage/schemes/curves/curve.py

@@ -340,7 +340,7 @@ def singular_points(self, F=None):
              (b^6 : -b^6 : 1)]
         """
         if F is None:
-            if not self.base_ring() in Fields():
+            if self.base_ring() not in Fields():
                 raise TypeError("curve must be defined over a field")
             F = self.base_ring()
         elif F not in Fields():

src/sage/schemes/elliptic_curves/cm.py

@@ -113,7 +113,7 @@ def hilbert_class_polynomial(D, algorithm=None):
     D = Integer(D)
     if D >= 0:
         raise ValueError("D (=%s) must be negative" % D)
-    if not (D % 4 in [0, 1]):
+    if (D % 4) not in [0, 1]:
         raise ValueError("D (=%s) must be a discriminant" % D)
 
     if algorithm == "arb":

src/sage/schemes/elliptic_curves/ell_number_field.py

@@ -2510,7 +2510,7 @@ def real_components(self, embedding):
             ValueError: invalid embedding specified: should have domain ...
         """
         try:
-            if not embedding.domain() is self.base_field():
+            if embedding.domain() is not self.base_field():
                 raise ValueError("invalid embedding specified: should have domain {}".format(self.base_field()))
             if not isinstance(embedding.codomain(), sage.rings.abc.RealField):
                 raise ValueError("invalid embedding specified: should be real")

src/sage/schemes/elliptic_curves/ell_point.py

@@ -1931,7 +1931,7 @@ def tate_pairing(self, Q, n, k, q=None):
         P = self
         E = P.curve()
 
-        if not Q.curve() is E:
+        if Q.curve() is not E:
             raise ValueError("Points must both be on the same curve")
 
         K = E.base_ring()
@@ -2139,7 +2139,7 @@ def ate_pairing(self, Q, n, k, t, q=None):
         # check for same curve
         E = P.curve()
         O = E(0)
-        if not Q.curve() is E:
+        if Q.curve() is not E:
             raise ValueError("Points must both be on the same curve")
 
         # set q to be the order of the base field

src/sage/schemes/elliptic_curves/ell_rational_field.py

@@ -4987,7 +4987,7 @@ def is_isogenous(self, other, proof=True, maxp=200):
         """
         if not is_EllipticCurve(other):
             raise ValueError("Second argument is not an Elliptic Curve.")
-        if not other.base_field() is QQ:
+        if other.base_field() is not QQ:
             raise ValueError("If first argument is an elliptic curve over QQ then the second argument must be also.")
 
         if self.is_isomorphic(other):

src/sage/schemes/elliptic_curves/heegner.py

@@ -118,14 +118,11 @@
 from sage.misc.verbose import verbose
 from sage.modular.modsym.p1list import P1List
 from sage.rings.complex_double import CDF
-from sage.rings.complex_mpfr import ComplexField
 from sage.rings.factorint import factor_trial_division
 from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
 from sage.rings.finite_rings.integer_mod_ring import IntegerModRing as Integers
 from sage.rings.integer_ring import ZZ
-from sage.rings.number_field.number_field import QuadraticField
 from sage.rings.rational_field import QQ
-from sage.rings.real_mpfr import RealField
 from sage.quadratic_forms.binary_qf import BinaryQF
 from sage.quadratic_forms.binary_qf import BinaryQF_reduced_representatives
 from sage.rings.number_field.number_field_element_base import NumberFieldElement_base

src/sage/schemes/elliptic_curves/isogeny_class.py

@@ -406,17 +406,17 @@ def graph(self):
         """
         from sage.graphs.graph import Graph
 
-        if not self.E.base_field() is QQ:
+        if self.E.base_field() is not QQ:
             M = self.matrix(fill=False)
             n = len(self)
             G = Graph(M, format='weighted_adjacency_matrix')
             D = dict([(v,self.curves[v]) for v in G.vertices(sort=False)])
             G.set_vertices(D)
-            if self._qfmat: # i.e. self.E.has_rational_cm():
+            if self._qfmat:  # i.e. self.E.has_rational_cm():
                 for i in range(n):
                     for j in range(n):
-                        if M[i,j]:
-                            G.set_edge_label(i,j,str(self._qfmat[i][j]))
+                        if M[i, j]:
+                            G.set_edge_label(i, j, str(self._qfmat[i][j]))
             G.relabel(list(range(1, n + 1)))
             return G
 

src/sage/schemes/elliptic_curves/period_lattice.py

@@ -1707,7 +1707,7 @@ def elliptic_logarithm(self, P, prec=None, reduce=True):
             sage: L.elliptic_logarithm(P, prec=1000)
             1.17058357737548897849026170185581196033579563441850967539191867385734983296504066660506637438866628981886518901958717288150400849746892393771983141354014895386251320571643977497740116710952913769943240797618468987304985625823413440999754037939123032233879499904283600304184828809773650066658885672885 - 1.13513899565966043682474529757126359416758251309237866586896869548539516543734207347695898664875799307727928332953834601460994992792519799260968053875387282656993476491590607092182964878750169490985439873220720963653658829712494879003124071110818175013453207439440032582917366703476398880865439217473*I
         """
-        if not P.curve() is self.E:
+        if P.curve() is not self.E:
             raise ValueError("Point is on the wrong curve")
         if prec is None:
             prec = RealField().precision()