switch to tomli for toml fallback

[graingert]

toml has been abandoned since Nov 1, 2020 and doesn't support some syntax

also tomli is used by pip and setuptools, so this project should match that

[dolfinus]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.72%. Comparing base (89b6727) to head (623256e).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   92.70%   92.72%   +0.01%     
==========================================
  Files           1        1              
  Lines         384      385       +1     
  Branches       59       59              
==========================================
+ Hits          356      357       +1     
  Misses         23       23              
  Partials        5        5              

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