new and handy functools.partialmethod doesn't fully support inspect.signature.

For instance, for the following code:

    class Spam:
        def say(self, a, b=1):
            print(a)
        hello = functools.partialmethod(say, 'hello')

the 'signature(Spam.hello)' will always return '(*args, **keywords)'

I'm attaching a patch that fixes that, so the signature for the above example will be '(self, b=1)'.