refactor: add "flake8-simplify" plugin and simplify code

[sisp]

I've added the Flake8 plugin flake8-simplify and applied its simplification suggestions.

There's one exception though: SIM117 which suggests merging several with statements that use the same scope. Applying this rule leads to some ugly formatting by Black (psf/black#412, psf/black#664).

I think this Flake8 plugin has identified some nice small improvements and contributes to maintaining a clean code base. Poetry uses it, too. 😉

[codecov-commenter]

Codecov Report

Merging #776 (7f8f03b) into master (13fddd9) will decrease coverage by 0.00%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #776      +/-   ##
==========================================
- Coverage   96.62%   96.61%   -0.01%     
==========================================
  Files          41       41              
  Lines        2930     2928       -2     
==========================================
- Hits         2831     2829       -2     
  Misses         99       99              

Flag	Coverage Δ
unittests	96.61% <80.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
copier/main.py	94.39% <75.00%> (-0.04%)	⬇️
copier/vcs.py	97.46% <100.00%> (ø)

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[yajo]

Thanks!