Revert "add back in Python 3.7 for one last release"

[mr-c]

This reverts commit 6c32ae5.

[codecov]

Codecov Report

Merging #1901 (541c120) into main (2e095e1) will increase coverage by 0.00%.
The diff coverage is 57.14%.

@@           Coverage Diff           @@
##             main    #1901   +/-   ##
=======================================
  Coverage   83.96%   83.97%           
=======================================
  Files          46       46           
  Lines        8172     8162   -10     
  Branches     2169     2168    -1     
=======================================
- Hits         6862     6854    -8     
+ Misses        841      838    -3     
- Partials      469      470    +1     

Files Changed	Coverage Δ
cwltool/context.py	100.00% <ø> (ø)
cwltool/docker.py	76.52% <0.00%> (-0.11%)	⬇️
cwltool/software_requirements.py	72.97% <50.00%> (-0.37%)	⬇️
cwltool/utils.py	84.52% <50.00%> (-0.24%)	⬇️
cwltool/checker.py	96.87% <100.00%> (-0.03%)	⬇️
cwltool/cwlprov/__init__.py	92.00% <100.00%> (-0.11%)	⬇️

... and 1 file with indirect coverage changes