Temporarily switch intersphinx to latest pytest.

The incorrect reference is fixed in pytest git, but won't be in a
released version for some time.
pytest-dev/pytest#8280 (comment)

doc/conf.py

@@ -141,7 +141,7 @@ def _check_dependencies():
     'ipykernel': ('https://ipykernel.readthedocs.io/en/latest/', None),
     'numpy': ('https://numpy.org/doc/stable/', None),
     'pandas': ('https://pandas.pydata.org/pandas-docs/stable/', None),
-    'pytest': ('https://pytest.org/en/stable/', None),
+    'pytest': ('https://pytest.org/en/latest/', None),
     'python': ('https://docs.python.org/3/', None),
     'scipy': ('https://docs.scipy.org/doc/scipy/reference/', None),
 }