Drop Python 3.7 support and run tests on latest patch version

[randomir]

• drop py37
• always run on the latest patch version of python
• migrate to new codecov uploader
• up the cloud-client dep to avoid pydantic2 break
• drop appveyor in favor of circleci

[codecov[bot]]

Codecov Report

Merging #499 (4ae62f4) into master (5cb4cee) will decrease coverage by 3.17%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
- Coverage   90.87%   87.70%   -3.17%     
==========================================
  Files          24       24              
  Lines        1676     1676              
==========================================
- Hits         1523     1470      -53     
- Misses        153      206      +53     

see 6 files with indirect coverage changes

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[randomir]

Oh, don't forget the appveyor badge. If we're removing it.

That's a good catch. Will remove it.