Fix coverage issues

[ktbarrett]

• Add fail_ci_on_error to ensure coverage upload failures aren't missed.
• Disable collection of coverage on RHEL8 since it isn't supported with the default uploader (xref RHEL8 coverage upload broken #70).
• Use use_pypi to install the codecov upload from pypi for the RHEL8 test with Python 3.9 since the pure Python source should work on RHEL8, but needs Python 3.9.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.51%. Comparing base (d93fd0a) to head (f26ef67).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #72   +/-   ##
=======================================
  Coverage   67.51%   67.51%           
=======================================
  Files           3        3           
  Lines         197      197           
  Branches       47       47           
=======================================
  Hits          133      133           
  Misses         48       48           
  Partials       16       16           

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