Further modernization, removing six

[bmcfee]

This PR continues the modernization effort by removing six and lingering references to python 2 support.

Once we're in the green, I'll also do some code style updates.

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (e4f6b68) 97.46% compared to head (4c90883) 97.80%.

Files	Patch %	Lines
pescador/mux.py	86.84%	5 Missing ⚠️
pescador/core.py	83.33%	1 Missing ⚠️
pescador/util.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   97.46%   97.80%   +0.33%     
==========================================
  Files           8        8              
  Lines         553      546       -7     
==========================================
- Hits          539      534       -5     
+ Misses         14       12       -2     

Flag	Coverage Δ
unittests	97.80% <91.25%> (+0.33%)	⬆️

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