Don't add names that are bound in only one branch of an undecidable if

[hakancelikdev]

From the core-dev review of the integration branch. The problem also exists on main.

Problem

if sys.platform == "win32":
    class WinOnly: ...

WinOnly was added to __all__, so from module import * raised AttributeError on every other platform. Stdlib modules such as ctypes, subprocess, selectors and multiprocessing.* follow this pattern.

Fix

• New relate.is_conditional_only and rule _rule_not_conditional_only.
• A name is not added when it is bound in only one branch of an if whose test can't be decided statically. Tests that can be decided (TYPE_CHECKING, the main guard, constants) are already handled by is_runtime_missing.
• elif chains are followed: each level must bind the name in the other branch. A binding can be an assignment, def/class or import.
• Names that are already listed are still kept. # unexport: public still adds a name explicitly.
• Documented in "Useful features" and the changelog.

This is a behavior change: a name defined only inside e.g. if HAS_NUMPY: is no longer added automatically. I chose this conservative default because adding it produces an __all__ that breaks star imports on the other branch. try/except ImportError bodies are left as they were.

Tests

• test_names_bound_in_one_branch_only covers if/elif/else binding in every branch, a def vs an assignment, an import in the other branch, and the # unexport: public override. It fails before this change.
• test_listed_conditional_name_is_kept covers an already-listed conditional name.
• 3.10 / 3.12 / 3.14, pre-commit, mypy and the self-check all pass.

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