What does this implement/fix? Explain your changes.

If a class A had indexer, and class B derived from it, Python.NET would
not consider class B to have any indexer.

Does this close any currently open issues?

No

Checklist

Check all those that are applicable and complete.

• Make sure to include one or more tests for your change
• If an enhancement PR, please create docs and at best an example
• Add yourself to AUTHORS
• Updated the CHANGELOG