cibuild matrix, code coverage and other enhancements

[bart-degreed]

Similar to the changes in json-api-dotnet/JsonApiDotNetCore#976.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@4e31a15). Click here to learn what that means.
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@@            Coverage Diff            @@
##             master      #11   +/-   ##
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  Coverage          ?   84.85%           
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  Files             ?       16           
  Lines             ?      284           
  Branches          ?        0           
=========================================
  Hits              ?      241           
  Misses            ?       43           
  Partials          ?        0           

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