Ci tweaks, poetry, etc

[tony]

• Deprecate python 3.6
• Add python 3.10
• Update black 21.9b0 -> 21.12b0
• Run formatters
• Add pre-commit-config.yaml with black, isort, flake8
• Remove unused sphinx-issues package

[codecov]

Codecov Report

Merging #726 (89ae436) into master (d0e86c0) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
- Coverage   75.40%   75.23%   -0.18%     
==========================================
  Files           8        8              
  Lines        1175     1175              
  Branches      295      299       +4     
==========================================
- Hits          886      884       -2     
- Misses        208      210       +2     
  Partials       81       81              

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
tmuxp/cli.py	72.98% <100.00%> (ø)
tmuxp/util.py	86.11% <0.00%> (-2.78%)	⬇️

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