functools.singledispatchmethod: add __call__ for better results (#3899)

It doesn't actually have __call__, it does some descriptor stuff, but
this makes things work. _SingleDispatchCallable has a __call__ too,
which is what this mirrors.

Fixes #3898

tests/stubtest_whitelists/py38.txt

@@ -97,6 +97,7 @@ dis.stack_effect
 email.message.MIMEPart.as_string
 enum.Enum._generate_next_value_
 functools.partialmethod.__get__
+functools.singledispatchmethod.__call__  # A lie to reflect that the descriptor get returns a callable
 gettext.install
 gettext.translation
 hmac.compare_digest