MAINT: Enforce pre-2.0-style reprs for numpy scalars.

This will silence doctest errors related to changes in the NumPy
scalar reprs per NEP 51.
The goal is to aid in transitioning to new numpy scalars with the
minimum possible churn in code.
In general, the transition plan would be something like:
 - Leave this PR in place to suppress doctest failures from
   changes in numpy scalar reprs. This way, all doctests can still
   be run.
 - Update / modify code and/or tests as needed to limit/replace the
  use of numpy scalars where appropriate.
 - When NumPy 2.0 becomes the minimum supported version for NetworkX,
   These 5 lines can be removed and any remaining issues related to
   scalar representations fixed (hopefully there aren't many left...
   see previous bullet).

networkx/conftest.py

@@ -94,6 +94,12 @@ def set_warnings():
 @pytest.fixture(autouse=True)
 def add_nx(doctest_namespace):
     doctest_namespace["nx"] = networkx
+    try:
+        import numpy as np
+
+        np.set_printoptions(legacy="1.21")
+    except ImportError:
+        pass
 
 
 # What dependencies are installed?