Remove Python 3.8 support

[mr-c]

As per https://cwltool.readthedocs.io/en/latest/pythonversions.html

Do not merge until Python 3.8 is officially declared to have reached its End of Life (expected by the end of this month, October 2024)

[codecov]

Codecov Report

Attention: Patch coverage is 97.23183% with 8 lines in your changes missing coverage. Please review.

Project coverage is 83.93%. Comparing base (83def6a) to head (0ac6018).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cwltool/cwlprov/provenance_profile.py	83.33%	2 Missing ⚠️
cwltool/main.py	86.66%	1 Missing and 1 partial ⚠️
cwltool/builder.py	91.66%	1 Missing ⚠️
cwltool/cwlrdf.py	66.66%	1 Missing ⚠️
cwltool/docker.py	87.50%	0 Missing and 1 partial ⚠️
cwltool/singularity.py	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2047      +/-   ##
==========================================
+ Coverage   83.88%   83.93%   +0.05%     
==========================================
  Files          46       46              
  Lines        8274     8300      +26     
  Branches     2102     1957     -145     
==========================================
+ Hits         6941     6967      +26     
- Misses        855      856       +1     
+ Partials      478      477       -1     

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