Update actions and testing

[elbeejay]

PR does a bunch of housekeeping:

• Removes pinned sphinx version
• Updates .md to .rst conversion package from the unsupported m2r to m2r2
• Pins docs build to ubuntu 3.9 as all docs dependencies aren't supported on 3.10 yet
• Reorganizes example-tests into classes
• Switches syntax of pytext.approx calls to be compatible with current versions

[codecov[bot]]

Codecov Report

Merging #30 (5a0bef5) into master (84c073e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   97.84%   97.84%           
=======================================
  Files           2        2           
  Lines         556      556           
=======================================
  Hits          544      544           
  Misses         12       12           

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