test tox with py39 on Github Actions CI

[basnijholt]

Description

Test Python 3.9 and move CI to Github Actions.
Actions have a better integration in GitHub (obviously) and are faster to start than Azure Pipelines.

Checklist

• Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• pytest passed

Type of change

Check relevant option(s).

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

[codecov-io]

Codecov Report

Merging #302 (fddcce8) into master (ae2ddf4) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
- Coverage   80.66%   80.64%   -0.03%     
==========================================
  Files          35       35              
  Lines        4623     4623              
  Branches      832      832              
==========================================
- Hits         3729     3728       -1     
  Misses        769      769              
- Partials      125      126       +1     

Impacted Files	Coverage Δ
adaptive/tests/test_cquad.py	91.61% <0.00%> (-0.65%)	⬇️

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