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.

Start by tracing mypy's existing per-module python_version configuration and how it handles imports guarded by sys.version_info. The issue names no files or tests; done would require a defined, tested approach for conditionally supporting modules that contain syntax unavailable in older Python versions.