fix: windows asyncio raises NotImplementedError

[waketzheng]

Fixes #582

[SaladDais]

Might be better to put the initial loop.add_signal_handler() in a try: ... except NotImplementedError: signals_supported = False and then skip the signal removal if signals aren't supported?

I think the inspect.getsource() approach may have issues if you're using a bundled interpreter with only bytecode for the stdlib files.