get_type_hints() should support functools.partial()

[Zac-HD]

Our get_type_hints() helper doesn't currently support functools.partial(), but I think it should - it's easy enough to detect, recurse on the wrapped callable, and then drop any bound-positional args (bound kwargs just assign a default value).

We should also consider upstreaming this fix into typing.get_type_hints(), since inspect.signature() does this already.