Merge further typing.py changes from upstream.
diff --git a/Lib/typing.py b/Lib/typing.py
index 05b65b7..557fa58 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -993,7 +993,10 @@
# This allows unparameterized generic collections to be used
# with issubclass() and isinstance() in the same way as their
# collections.abc counterparts (e.g., isinstance([], Iterable)).
- self.__subclasshook__ = _make_subclasshook(self)
+ if ('__subclasshook__' not in namespace and extra # allow overriding
+ or hasattr(self.__subclasshook__, '__name__') and
+ self.__subclasshook__.__name__ == '__extrahook__'):
+ self.__subclasshook__ = _make_subclasshook(self)
if isinstance(extra, abc.ABCMeta):
self._abc_registry = extra._abc_registry
return self