Support conditionally importing from typing_extensions

This code tries to import TypeAlias from typing if possible and falls back to typing_extensions if it isn't available (e.g. on Python 3.8):

try:
    from typing import TypeAlias  # type: ignore
except ImportError:
    from typing_extensions import TypeAlias

Mypy doesn't actually support this as expected in older Python versions that are missing the definition in typing. They will just get an Any value, and the except block has no significance.

Perhaps the import from typing_extensions should take precedence? Or can we at least generate a more specific error about this?

A possible workaround is to flip the order of the imports.

This probably affects any feature that requires an import of a backported definition from typing_extensions.

Related issue: #14219

Non viene indicato alcun file, test o punto di ingresso. Inizia esaminando il comportamento di importazione condizionale di TypeAlias descritto qui e la issue correlata #14219; determina se è prevista una precedenza di importazione o una diagnosi, quindi verifica il comportamento scelto per le versioni precedenti di Python.