What does this implement/fix? Explain your changes.

It should have not been allowed in the first place, but we accidentally cleared Ready from tp_flags. Instead, we will extend tp_flags and call PyType_Modified at the end of CLR MetaType.tp_new

Does this close any currently open issues?

No

Any other comments?

This change is required for the upcoming feature, that allows setting custom base type for reflected .NET classes (as seen from Python).