Issue #12: sync module from CPython 3.10

[ncoghlan]

• Sync module and test suite from CPython 3.10
• Switch test suite over to using unittest autodiscovery (since there are now two test files)
• Capture the differences from the Python 3.10 version as a patch file
• Ship the tox config (and other dev files) in the sdist

[codecov]

Codecov Report

Merging #32 (032662d) into master (94f3881) will increase coverage by 2.99%.
The diff coverage is 95.63%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   92.34%   95.34%   +2.99%     
==========================================
  Files           1        1              
  Lines         196      365     +169     
  Branches       14       29      +15     
==========================================
+ Hits          181      348     +167     
- Misses         13       14       +1     
- Partials        2        3       +1     

Impacted Files	Coverage Δ
contextlib2.py	95.34% <95.63%> (+2.99%)	⬆️

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[graingert]

Might also be good to pull in the typeshed contextlib.pyi file

[ncoghlan]

@graingert That's a good idea, but could you file a new ticket for it? IIRC, there's additional package metadata that has to be set to allow tools to find it properly.