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 👍