Trac #31012: Modify RandomChordalGraph according a deprecation warning introduced in Python 3.9

Python 3.9 deprecates the use `sample(population, k)` when `population`
is a `set`.
{{{
sage: G = graphs.RandomChordalGraph(100)
/Users/dcoudert/sage/local/lib/python3.9/site-
packages/sage/misc/prandom.py:179: DeprecationWarning: Sampling from a
set deprecated
since Python 3.9 and will be removed in a subsequent version.
  return _pyrand().sample(population, k)
}}}
It was used in `graphs.RandomChordalGraph(...)`, so we change that.

URL: https://trac.sagemath.org/31012
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/graphs/generators/random.py

@@ -867,7 +867,7 @@ def growing_subtrees(T, k):
         sage: len(S)
         10
     """
-    from sage.misc.prandom import sample
+    from sage.misc.prandom import choice
     n = T.order()
     S = []
     for _ in range(n):
@@ -880,7 +880,7 @@ def growing_subtrees(T, k):
             neighbors = set(T.neighbor_iterator(x))
             for j in range(ki - 1):
                 # Select a random neighbor z outside of Ti and add it to Ti
-                z = sample(neighbors, 1)[0]
+                z = choice(tuple(neighbors))
                 Ti.add(z)
                 neighbors.update(y for y in T.neighbor_iterator(z) if y not in Ti)
             Vi = frozenset(Ti)