Enable `mypy` `union-attr` error code

[wochinge]

Maxime recently enabled mypy support for the first couple of issues. In our current setup we ignore certain typing issues as there are currently too many to tackle them all at once.

We should instead tackle them step by step. Related issue. Parent issue.

Current number of issues with this type: 91

Definition of Done:

• enable mypy checks for [union-attr]
• all existing [union-attr] issues are fixed

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.