Update ci.yml: go back to 3.9 since snakemake does not work with later versions

[smilesun]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5043785) 95.05% compared to head (b167fbc) 95.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #745      +/-   ##
==========================================
- Coverage   95.05%   95.03%   -0.03%     
==========================================
  Files         125      125              
  Lines        4911     4910       -1     
==========================================
- Hits         4668     4666       -2     
- Misses        243      244       +1     

Flag	Coverage Δ
unittests	95.03% <ø> (-0.03%)	:arrow_down:

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