Apply get_args fix from bpo-40398 to typing_extensions

[avalentino]

Applies the fix from https://bugs.python.org/issue40398 to typing_extensions: typing.get_args() now always returns an empty tuple for special generic aliases.

[the-knights-who-say-ni]

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[gvanrossum]

Seems this was already done in python/cpython#19857, I presume this PR was submitted by mistake.

[avalentino]

Actually I need the fix in typing_extensions otherwise I can't use get_args in Python 3.7.

[gvanrossum]

Okay, reopening. And next time please be a little more clear in what you want. Now go sign that CLA.

[gvanrossum]

Okay, now that I have the right context this is fine.

[avalentino]

Thanks @gvanrossum