style: switch pycodestyle -> flake8

[Nytelife26]

This relates to...

CI failure on #1195 and our linting system.

Rationale

pycodestyle is very out of date, and doesn't catch everything.
Furthermore, it is prone to errors and cannot read our
pyproject.toml file. flake8 is a modern alternative, also
produced by the Python Code Quality Authority.

Changes

• switch pycodestyle -> flake8 in ci
• update run_ci_checks.sh
• apply new linting
• finalize port to py3

Features

N/A.

Bug Fixes

N/A.

Breaking Changes and Deprecations

N/A.

[codecov]

Codecov Report

Merging #1198 (ecf06d6) into main (0816a5b) will decrease coverage by 0.38%.
The diff coverage is 98.94%.

@@            Coverage Diff             @@
##             main    #1198      +/-   ##
==========================================
- Coverage   94.72%   94.34%   -0.39%     
==========================================
  Files          83       82       -1     
  Lines        1213     1131      -82     
==========================================
- Hits         1149     1067      -82     
  Misses         64       64              

Flag	Coverage Δ
macos-latest	94.34% <98.94%> (-0.39%)	⬇️
py3.6	93.74% <98.94%> (-0.47%)	⬇️
py3.7	93.74% <98.94%> (-0.47%)	⬇️
py3.8	94.34% <98.94%> (-0.39%)	⬇️
py3.9	94.34% <98.94%> (-0.39%)	⬇️
pypypy3	93.74% <98.94%> (-0.47%)	⬇️
ubuntu-latest	94.34% <98.94%> (-0.39%)	⬇️
windows-latest	94.34% <98.94%> (-0.39%)	⬇️

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Impacted Files	Coverage Δ
proselint/checks/consistency/spacing.py	100.00% <ø> (ø)
proselint/checks/consistency/spelling.py	100.00% <ø> (ø)
proselint/checks/hedging/misc.py	100.00% <ø> (ø)
proselint/checks/lgbtq/terms.py	100.00% <ø> (ø)
proselint/checks/misc/back_formations.py	100.00% <ø> (ø)
proselint/checks/misc/capitalization.py	100.00% <ø> (ø)
proselint/checks/misc/commercialese.py	100.00% <ø> (ø)
proselint/checks/misc/inferior_superior.py	100.00% <ø> (ø)
proselint/checks/misc/institution_name.py	100.00% <ø> (ø)
proselint/checks/misc/latin.py	100.00% <ø> (ø)
... and 70 more

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