Support `typing.get_type_hints(partial(fn, ...))` by 'unwrapping' the `partial`, and discarding hints for bound arguments

We added this to Hypothesis' get_type_hints() helper a few years ago, and while most of that function is clearly not suitable to upstream, it struck me recently that "works with functools.partial" could quite reasonably be added to the stdlib function.

It's also plausible that the typing module prefers to avoid calling inspect.signature, though I don't have a great sense of how to determine which parameters have been bound without getting the signature of the partial object. In any case, the implementation need not be the same as in Hypothesis, although as the author you're welcome to use that code if you'd like.

Start by reading the existing typing.get_type_hints implementation and the referenced Hypothesis helper in hypothesis-python/src/hypothesis/internal/compat.py. Check how functools.partial and inspect.signature expose bound arguments, then define tests showing that partials are unwrapped and hints for bound arguments are discarded. Done means the behavior is covered without breaking existing get_type_hints cases.