Improve Python <-> .NET exception integration

[lostmsu]

What does this implement/fix? Explain your changes.

1. .NET exceptions bubbled through Python stack back into .NET are preserved (before: PythonException was thrown instead).
2. Codecs can now encode and decode exceptions
3. .NET InnerException property is populated from Python's __cause__ attribute.
4. Python exception stack is added to .NET exception stack
5. Internal: simplified memory management in PythonException class. Instead of working with raw pointers, uses PyObject instances.

Breaking

1. Traceback, PyVal and PyType are now stored and returned as PyObjects.
2. PythonException.Restore no longer nulls out PythonException instance.
3. PythonException instances are no longer disposable. You can still dispose .Type, .Value and .Traceback, but it is not recommended, as they may be shared with other instances.

Does this close any currently open issues?

#893, #1098

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

[lostmsu]

Damn CI failures do not reproduce locally :/

[Felk]

Hello, given that Exceptions is not public anymore, what is the recommended way of catching specific python exceptions? Previously I used this:

catch (PythonException ex) when (ex.PyType == Exceptions.KeyError)

[lostmsu]

@Felk You can get KeyError from builtins module.

[Felk]

Thank you, this is how I got it to work now:

dynamic builtins = Py.Import("builtins");
try
{
    // ...
}
catch (PythonException ex) when (ex.Type.Equals(builtins.KeyError))
{
    // ...
}

[lostmsu]

@Felk comparing type handles will be slightly faster

[Felk]

@lostmsu can you give me an example of what you mean?

[lostmsu]

@Felk ex.Type.Handle == builtins.KeyError.Handle. But in you scenario that should not matter, cause generally exception handling code won't be performance critical part.

If you really cared, you'd also cache builtins.KeyError into a variable of type PyObject to avoid dynamic dispatch.