PY-25885 Remove registered sphinxcontrib modules in docstring_formatter.py

Otherwise their presense doesn't let us import sphinxcontrib.napoleon
that we need to render docstrings in Numpy/Google Style formats.

It's not enough to remove site-packages/dist-packages from sys.path to
isolate the script from such "malicious" packages, since they are
registered and installed in sys.modules on interpreter startup before
any path modifications happen.

Launching interpreter with -S option prevents it from running these
hooks and also excludes site-packages from sys.path but makes inherited
stdlib modules not available in virtualenv.

What's more, sphinxcontrib.napoleon itself depends on setuptools as it
tries to register itself in a similar fashion in its __init__.py.
We could patch it but that would make future updates of the library
more tedious and error-prone.

python/helpers/docstring_formatter.py

@@ -369,6 +369,11 @@ def format_epytext(docstring):
 
 
 def main():
+    # Remove existing Sphinx extensions registered via
+    # sphinxcontrib setuptools namespace package, as they
+    # conflict with sphinxcontrib.napoleon that we bundle.
+    sys.modules.pop('sphinxcontrib', None)
+
     args = sys.argv[1:]
 
     docstring_format = args[0] if args else 'rest'
@@ -393,4 +398,8 @@ def main():
 
 
 if __name__ == '__main__':
-    main()
+    try:
+        main()
+    except ImportError:
+        print_safe('sys.path = %s\n\n' % sys.path, error=True)
+        raise