Drop Python 2 support in third-party stubs (#7703)

CONTRIBUTING.md

@@ -159,8 +159,6 @@ supported:
   When the stubs are updated to a newer version
   of the library, the version of the stub should be bumped (note that
   previous versions are still available on PyPI).
-* `python2` (default: `false`): If set to `true`, the top-level stubs
-  support both Python 2 and Python 3.
 * `requires` (optional): A list of other stub packages or packages with type
   information that are imported by the stubs in this package. Only packages
   generated by typeshed or required by the upstream package are allowed to