drop six requirement

[branchvincent]

I noticed six is still required/used even though only 3.6+ is supported

• First commit is all from pyupgrade: pipx run pyupgrade --py36-plus $(git ls-files '*.py')
• Second commit is manual fixes, replacing the remaining uses of six and updating requirements

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (03c7993) 89.45% compared to head (e44b6ee) 89.44%.

Files	Patch %	Lines
shub/utils.py	80.00%	3 Missing ⚠️
shub/compat.py	60.00%	2 Missing ⚠️
shub/copy_eggs.py	33.33%	2 Missing ⚠️
shub/config.py	88.88%	1 Missing ⚠️
shub/exceptions.py	50.00%	1 Missing ⚠️
shub/schedule.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #438      +/-   ##
==========================================
- Coverage   89.45%   89.44%   -0.02%     
==========================================
  Files          34       34              
  Lines        2438     2407      -31     
==========================================
- Hits         2181     2153      -28     
+ Misses        257      254       -3     

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[Gallaecio]

❤️

[branchvincent]

btw, any interest in raising the minimum python to 3.8 (since older versions are EOL)?

[wRAR]

Yeah, I don't think it's useful to keep support for pre-3.8.

[branchvincent]

Rebased to resolve conflicts after #434