PEP 621 + src/ layout

[jhunkeler]

This PR does the following:

• The project directory costools was moved to src/costools
• setup.py has been replaced by pyproject.toml. The script entry points have been carried over
• The defunct azure-pipelines.yml configuration was removed
• The pypi upload workflow has been updated to use OpenAstronomy's reusable workflow. The wheel/sdist build will kick off for each PR and push to master. Uploads to pypi only occur when a release object is published
• Dependabot has been added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 11.12%. Comparing base (58422d3) to head (a2ed935).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   10.94%   11.12%   +0.18%     
==========================================
  Files           7        8       +1     
  Lines         987      989       +2     
  Branches      195      195              
==========================================
+ Hits          108      110       +2     
  Misses        879      879              

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[stscirij]

I think the only reason this needs to be in the build is because of the add_cos_s_region script/module. I tested installing from this and running the script and it seems to work OK.