What does this implement/fix? Explain your changes.

This PR addresses these issues:

• Current pythonnet does not allow subclassing from CLR Abstract classes
• Running python script with types containing __namespace__, more than once, would throw Type Exists exception
• Calling super().VirtualMethod() inside a python-derived virtual method, would result in an infinite loop

Due to the nature of the changes, this is a single PR instead of multiple smaller ones.

After PR merge:

• Python types can derive from CLR abstract classes. As before, abstract methods that are not implemented on the python type, will dynamically throw NotImplemented exception.
• Specifying __namespace__ is not required to created a ClassDerived anymore. All python derived classes are now represented by ClassDerived in managed memory.
• ClassDerived now caches generated types based on namespace, type name, and chain of base classes. This avoids regenerating types or throwing Type Already Exists exceptions. A new format for generated type names include full name of base class and interfaces in the typename. The naming format still ends in the python type name to be backward compatible and passes the unit tests
  • Example: Full name of derived clr type for python type named SubClass, deriving from BaseClass and implementing BaseInterface1 and BaseInterface2:
    Python.Runtime.Dynamic.BaseClass__BaseInterface1__BaseInterface2__main__SubClass
• Support for one base class and multiple interfaces (Merged some ideas and code from 1783 Implement Interface And Inherit Class #2028 - Potential merge conflicts)
• Derived class can call chosen constructors using super().__init__() pattern.

class SubClass(BaseClassA):
   def __init__(self, v):
       super().__init__(v)

• Derived class can call base virtual methods using super().method() pattern.

class SubClass(BaseClassA):
    def DoWorkWith(self, value):
        r = super().DoWorkWith(value) + 10
        return r

• Improved virtual method and getter/setter routing. Custom attributes (OriginalMethod and RedirectedMethod) are now used to mark the original and redirected virtual methods. The method name format for OriginalMethod is changed to $"_BASEVIRTUAL__{name}" so it can handle calling original methods on base classes that does not match the name of current class. Previously this was not working.
• Other misc refactoring and improvements. No change in behavior.

Does this close any currently open issues?

Potentially (no tests have been done to ensure these issues are resolved)

• Deriving from .NET types in Python does not inherit constructor #1945
• Derived class in Python not being called by C# #1107
• Can not call inherited method on nested class, when another nested class inherits from it and defines an override. #1787

Any other comments?

...

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
• Ensure you have signed the .NET Foundation CLA
• Add yourself to AUTHORS
• Updated the CHANGELOG