Use typevar with values rather than overloading in 'struct' stubs

[JukkaL]

It seems that all overloads in stubs/3.2/struct.py can be refactored to use AnyStr, which would be much cleaner.

[refi64]

Toying with this, what should I do with:

@overload
def unpack_from(fmt: str, buffer: Any) -> Any: pass
@overload
def unpack_from(fmt: bytes, buffer: Any, offset: int = 0) -> Any: pass

The second function takes an extra parameter. What should be done?

[JukkaL]

The current stub actually seems to be too restrictive. offset is always valid. This seems like a good signature:

unpack_from(fmt: Union[str, bytes], buffer: Any, offset: int = 0) -> Any: pass