Conditionally excluding backward incompatible syntax based on python version

Feature

There doesn't seem to be a way to progressively provide improvements in libraries that support older versions with syntactically backward incompatible code that is conditionally included at runtime.

For example in pythons < 3.12 it's not possible to exclude certain files containing PEP 695 type syntax even if the module is only imported with a sys.version_info >= (3,12) guard.

import sys

if sys.version_info >= (3, 12):
    from _py312_types import A
else:
    from _types import A

__all__ = ["A"]

# file: _py312_types.py
type A = list[int|A]

# file: _types.py
A = list[int|list["A"]]

Pitch

The only existing path that I could find was to allow python_version overrides on a per module basis (which is currently only acceptable as a global configuration) though that seems hacky and brittle.

Inizia tracciando la configurazione esistente di python_version per modulo di mypy e il modo in cui gestisce gli import protetti da sys.version_info. L'issue non indica file o test; completarla richiederebbe un approccio definito e testato per supportare condizionalmente i moduli che contengono sintassi non disponibile nelle versioni precedenti di Python.