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.

先阅读现有的 typing.get_type_hints 实现，以及 hypothesis-python/src/hypothesis/internal/compat.py 中引用的 Hypothesis 辅助函数。检查 functools.partial 和 inspect.signature 如何暴露已绑定的参数，然后定义测试，证明 partial 会被解包，并且已绑定参数的类型提示会被丢弃。完成的标准是在不破坏现有 get_type_hints 用例的情况下覆盖该行为。