Backport get_origin() and get_args()

[jstasiak]

The implementations come from CPython commit 427c84f13f77 with one small
change – the get_origin's docstring mentions Annotated as it's also
supported.

get_origin() and get_args() introduced in [1] and modified in [2] to
support Annotated.

[1] python/cpython#13685
[2] python/cpython#18260

[jstasiak]

Well, I'm not sure what to do regarding the build failure:

_____________ AllTests.test_typing_extensions_defers_when_possible _____________

self = <test_typing_extensions.AllTests testMethod=test_typing_extensions_defers_when_possible>

    def test_typing_extensions_defers_when_possible(self):

        exclude = {

            'overload',

            'Text',

            'TypedDict',

            'TYPE_CHECKING',

            'Final',

            'get_type_hints'

        }

        for item in typing_extensions.__all__:

            if item not in exclude and hasattr(typing, item):

>               self.assertIs(

                    getattr(typing_extensions, item),

                    getattr(typing, item))

E               AssertionError: <function get_args at 0x7fcb84063430> is not <function get_args at 0x7fcb8a0b3c10>

typing_extensions/src_py3/test_typing_extensions.py:1838: AssertionError

It happens because even though 3.8 has get_args and get_origin they don't support PEP 593 Annotated (only Python 3.9 get_origin and get_args do), so I redefine them in typing_extensions on 3.8 anyway, it's a bit of a mess. Is it acceptable to add them to the exclude list?

[gvanrossum]

Yeah, it looks like it belongs in the exclude list (maybe only in 3.8.x?)

[gvanrossum]

sys.version_info[:2] >= (3, 9)

[gvanrossum]

LGTM. I'd wait a few days for @ilevkivskyi -- Ivan, if you're okay with my review just say so and I'll merge it.

[ilevkivskyi]

LGTM 👍