Remove Python 3.7 support and add a new action for mamba.

[tobiasraabe]

Blocked until Python 3.7 is deprecated.

mamba has deprecated 3.7 some time ago and now it would be cumbersome to work around this.

[codecov]

Codecov Report

Merging #323 (8baac23) into main (39bfcdf) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 8baac23 differs from pull request most recent head 2f05ae7. Consider uploading reports for the commit 2f05ae7 to get more accurate results

@@            Coverage Diff             @@
##             main     #323      +/-   ##
==========================================
+ Coverage   96.16%   96.21%   +0.04%     
==========================================
  Files          93       93              
  Lines        7797     7837      +40     
==========================================
+ Hits         7498     7540      +42     
+ Misses        299      297       -2     

Flag	Coverage Δ
end_to_end	82.77% <100.00%> (+0.11%)	⬆️
integration	41.27% <100.00%> (+0.32%)	⬆️
unit	66.79% <100.00%> (+0.19%)	⬆️

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Impacted Files	Coverage Δ
tests/test_debugging.py	89.89% <ø> (-0.04%)	⬇️
src/_pytask/capture.py	90.23% <100.00%> (+0.47%)	⬆️
src/_pytask/nodes.py	97.18% <100.00%> (+0.75%)	⬆️
src/_pytask/path.py	95.52% <100.00%> (ø)

... and 9 files with indirect coverage changes