CONTRIBUTING.md: recommend updating pip in the dev venv

[simleo]

I tried the "traditional workflow" described in CONTRIBUTING.md but got the following error:

ERROR: Double requirement given: ruamel.yaml<0.17.22,>=0.16.0 (from -r mypy-requirements.txt (line 2)) (already in ruamel.yaml<0.17.22,>=0.15 (from -r requirements.txt (line 3)), name='ruamel.yaml')

That was with pip 20.0.2. I solved by updating pip to the latest version, hence this PR.

[kinow]

Thanks!

[codecov]

Codecov Report

Merging #1726 (6d9b9c3) into main (6f29c59) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1726      +/-   ##
==========================================
+ Coverage   66.56%   66.60%   +0.03%     
==========================================
  Files          89       89              
  Lines       15846    15846              
  Branches     4188     4188              
==========================================
+ Hits        10548    10554       +6     
+ Misses       4207     4203       -4     
+ Partials     1091     1089       -2     

Impacted Files	Coverage Δ
cwltool/cwltool/singularity.py	51.94% <0.00%> (+0.43%)	⬆️
cwltool/cwltool/job.py	62.45% <0.00%> (+0.98%)	⬆️

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