Improve error message for module-level __getattr__ issues

[veeceey]

Summary

Fixes #8265

This PR improves the error message when a module-level __getattr__ returns None instead of raising AttributeError.

Problem

When a module defines a custom __getattr__ that returns None instead of raising AttributeError, pytest gives a cryptic error message:

TypeError: got None instead of Mark

This is confusing for users who don't understand what a "Mark" is or why pytest is looking for one.

Solution

Added a special check in get_unpacked_marks() to detect when the pytestmark attribute resolves to None and emit a PytestCollectionWarning that explains:

1. What the likely cause is (module-level __getattr__)
2. What the fix is (raise AttributeError)

Changes

• Modified get_unpacked_marks() in /src/_pytest/mark/structures.py to detect None and emit a warning
• Added test case to verify the new warning message
• Added changelog entry

Test plan

• Added test in testing/test_mark.py to verify new warning message
• Manually verified with reproduction case from Cryptic Error Message when module-level __getattr__ fails to raise AttributeError #8265

[RonnyPfannschmidt]

This is the wrong place for the check there should be a warning triggered in the helper to get that list

[veeceey]

Thanks for the review @RonnyPfannschmidt! You're right — the check was in the wrong place.

I've moved it from normalize_mark_list() to get_unpacked_marks(), which is the helper that actually fetches the pytestmark attribute list. Now when a module-level __getattr__ returns None instead of raising AttributeError, a PytestCollectionWarning is emitted (instead of raising a TypeError), and the module continues to be collected normally.

Changes in the latest push:

• Moved the None check to get_unpacked_marks() where getattr(obj, "pytestmark", []) is called
• Changed from raising TypeError to emitting a PytestCollectionWarning
• Updated the test and changelog accordingly