Identity checks of `Literal` objects inconsistent between Python <3.10 and 3.10<=

[rsokl]

import typing
import typing_extensions as tp_ext

assert typing.Literal['a'] is typing.Literal['a']  # Passes for  Python 3.8<=
assert tp_ext.Literal['a'] is tp_ext.Literal['a']   # Fails for Python <3.10

[JelleZijlstra]

This isn't a guaranteed part of the interface, just as you may see differences in behavior for checks like 1 is 1 between Python versions.