What does this implement/fix? Explain your changes.

Implemented by adding tp_call to ClassBase, that uses reflection to find __call__ methods in .NET, and falls back to invoking __call__ method from Python base classes.

Does this close any currently open issues?

Implements #890

Any other comments?

This is an amalgamation of d46878c, 5bb1007, and 960457f from https://github.com/losttech/pythonnet

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
• Updated the CHANGELOG