native except handler API

[maxfischer2781]

This PR is a prototype implementation of #20. Notable additions and changes include:

• ExceptionGroup is "Generic" and can be specialised by exceptions. This allows catching specific groups, e.g. except ExceptionGroup[KeyError, TypeError] as exc_group:.
• Added new metaclass to implement ExceptionGroup[A, B, C] and issubclass(exc_group, ExceptionGroup[ABC]).
• ExceptionGroup attributes .exceptions and .sources are tuples instead of lists. This is intended to prevent inconsistencies between the type and its attributes.

This PR does not change:

• ExceptionGroup having .message, .exceptions and .sources attributes.
• A bare except ExceptionGroup: catching all possible ExceptionGroups.
• Existing API for splitting/catching sub-exceptions.
• There is no "sub-handler" API as part of this PR.

[codecov]

Codecov Report

Merging #21 into master will increase coverage by 3.32%.
The diff coverage is 82.85%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   66.55%   69.88%   +3.32%     
==========================================
  Files          11       12       +1     
  Lines         311      425     +114     
  Branches       28       49      +21     
==========================================
+ Hits          207      297      +90     
- Misses        101      115      +14     
- Partials        3       13      +10     

Impacted Files	Coverage Δ
exceptiongroup/_tests/test_exceptiongroup.py	88.76% <73.68%> (-11.24%)	⬇️
exceptiongroup/_exception_group.py	85.85% <85.85%> (ø)
exceptiongroup/__init__.py	100.00% <100.00%> (ø)
exceptiongroup/_monkeypatch.py	31.25% <100.00%> (ø)
exceptiongroup/_tools.py	56.06% <100.00%> (ø)

[maxfischer2781]

As a first draft, this is basically done. Let me know if there is anything else that I should add, or something that needs clarification or discussing.