Fix type hints when "from __future__ import annotations" is included

[hjtran]

If a module imports from __future__ imports annotations, any function we try to get type hints from using typing.decorators.get_signature returns Any. This is because the future import causes all typehints to become strings (to postpone evaluation) and get_signature doesn't know how to resolve postponed typehint strings.

This diff extends get_signature so it actually resolves these annotations.

I tried to keep the get_signature behavior consistent between both modes (import from future and not importing from future), so I just created a generation script to clone the typehints_test and decorators_test but with the future import.

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