autodoc: blacklist typing.Generic.__new__

When documenting classes derived from typing.Generic (essentially all classes in the
typing module) the constructor signature would show an unhelpful (*args, **kwds).

typing.Generic has a __new__ method which was picked up by sphinx. With this patch it
is skipped and constructor signatures for generic classes are shown as they should.

sphinx/ext/autodoc/__init__.py

@@ -1317,6 +1317,12 @@ def format_args(self, **kwargs: Any) -> Any:
 ]
 
 
+# Types whose __new__ signature is a pass-thru.
+_CLASS_NEW_BLACKLIST = [
+    'typing.Generic.__new__',
+]
+
+
 class ClassDocumenter(DocstringSignatureMixin, ModuleLevelDocumenter):  # type: ignore
     """
     Specialized Documenter subclass for classes.
@@ -1385,6 +1391,11 @@ def get_user_defined_function_or_method(obj: Any, attr: str) -> Any:
 
         # Now we check if the 'obj' class has a '__new__' method
         new = get_user_defined_function_or_method(self.object, '__new__')
+
+        if new is not None:
+            if "{0.__module__}.{0.__qualname__}".format(new) in _CLASS_NEW_BLACKLIST:
+                new = None
+
         if new is not None:
             self.env.app.emit('autodoc-before-process-signature', new, True)
             try:

tests/test_ext_autodoc.py

@@ -291,7 +291,6 @@ def foo3(self, d='\n'):
 
 
 @pytest.mark.skipif(sys.version_info < (3, 5), reason='typing is available since python3.5.')
-@pytest.mark.xfail
 def test_autodoc_process_signature_typing_generic(app):
     import typing