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.

先閱讀現有的 typing.get_type_hints 實作，以及 hypothesis-python/src/hypothesis/internal/compat.py 中引用的 Hypothesis 輔助函式。檢查 functools.partial 和 inspect.signature 如何公開已繫結的引數，然後定義測試，證明 partial 會被解包，且已繫結引數的型別提示會被捨棄。完成的標準是在不破壞現有 get_type_hints 案例的情況下涵蓋該行為。