Set up automatic code-coverage report for backend

[VFermat]

Fixes #603

Generates automatic Code Coverage Report using pytest-cov and Coverage

Technical details
Alters the pytest workflow to generate a Coverage report using xml and send it to Coverage.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

Developer Certificate of Origin

Developer Certificate of Origin

Developer Certificate of Origin
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Everyone is permitted to copy and distribute verbatim copies of this
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Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
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(c) The contribution was provided directly to me by some other
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Vitor Eller

[VFermat]

Checked this as Ready for Review since CodeCov can't be tested on forks.

The workflow should run on this Pull Request and with that we can check if it is working properly!

[kgodey]

Looks like the coverage report did not upload: https://github.com/centerofci/mathesar/runs/4246068249?check_suite_focus=true

[kgodey]

@VFermat looks like the upload is still failing: https://github.com/centerofci/mathesar/runs/4258040017?check_suite_focus=true

[github-actions]

This pull request has not been updated in 45 days and is being marked as stale. It will automatically be closed in 30 days if not updated by then.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@8e022b3). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #817   +/-   ##
=========================================
  Coverage          ?   97.15%           
=========================================
  Files             ?      162           
  Lines             ?     7852           
  Branches          ?        0           
=========================================
  Hits              ?     7629           
  Misses            ?      223           
  Partials          ?        0           

Flag	Coverage Δ
pytest-backend	97.15% <0.00%> (?)

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[mathemancer]

I unfortunately needed to start a branch in this repo to finish this up. @VFermat thank you very much for your work on this. @kgodey Please continue further review / commenting on the new PR: #966