Issue 38: Fix failed test due to CPython str/enum behavior change

[jeremyfell]

Fixes a test failure that was due to the CyclingStatus end status of an experiment being printed as the enum name (i.e. CyclingStatus.LOW_CAPACITY) instead of the enum's string value (i.e. capacity is less than 1% of initial CLS capacity).

This failure is due to a behavior change between versions 3.10 and 3.11 of CPython, where the __format__ behavior of a class inheriting both str and Enum was changed (from returning the enum value to returning the enum name in 3.11+). See this CPython issue for further details.

This PR also updates the GitHub actions to run the tests using multiple versions of Python. Previously, only 3.10 was tested which is why this failure was not encountered on previous PRs.

This addresses #38, and relates to openjournals/joss-reviews#6537.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.90%. Comparing base (f6852fe) to head (8d2fa59).

Files	Patch %	Lines
src/rfbzero/experiment.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #42   +/-   ##
=======================================
  Coverage   94.90%   94.90%           
=======================================
  Files           4        4           
  Lines         608      608           
=======================================
  Hits          577      577           
  Misses         31       31           

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[ericfell]

All looks good LGTM