Use ast.unparse rather than ast.dump

[gordonwatts]

• ast.dump makes for a much more user-friendly error message. Replace ast.dump.
• Fix up a few tests that broke as a result of specific text no longer being present in error messages.
• Forces us to leave behind python 3.8 support.

Fixes #240

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.02%. Comparing base (1903b3a) to head (33366f3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #250   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files          35       35           
  Lines        2009     2009           
  Branches      221      221           
=======================================
  Hits         1909     1909           
  Misses         64       64           
  Partials       36       36           

Flag	Coverage Δ
unittests	95.02% <ø> (ø)

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