Add mypy ignore comments for Python 3.14 module imports

Fixes #13240

Use `sys.version_info >= (3, 14)` instead of the `py314` variable for
the py314-only imports block (`annotationlib`, `string.templatelib`).

mypy has built-in support for `sys.version_info` guards and only
type-checks the reachable branch, while a plain variable like `py314`
causes mypy to check both branches — leading to `import-not-found` on
<3.14 and `unused-ignore` on 3.14 if `type: ignore` comments are added.

Closes: #13239
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13239
Pull-request-sha: f58bfc11694c71020b98ed69d8400a0173be096b

Change-Id: Ib5e2361e849a3f14f90d0da34800c1d31f8bf201