Add flake8-quotes to the linter

[Riolku]

Also fix all existing quoting issues

[codecov-commenter]

Codecov Report

Merging #1763 (c22c58d) into master (56c111b) will not change coverage.
The diff coverage is 34.37%.

@@           Coverage Diff           @@
##           master    #1763   +/-   ##
=======================================
  Coverage   46.20%   46.20%           
=======================================
  Files         223      223           
  Lines       12674    12674           
=======================================
  Hits         5856     5856           
  Misses       6818     6818           

Impacted Files	Coverage Δ
dmoj_install_pymysql.py	0.00% <0.00%> (ø)
judge/admin/runtime.py	66.66% <ø> (ø)
judge/contest_format/atcoder.py	30.98% <0.00%> (ø)
judge/contest_format/ecoo.py	28.57% <ø> (ø)
judge/contest_format/icpc.py	28.75% <0.00%> (ø)
judge/contest_format/ioi.py	26.82% <0.00%> (ø)
judge/contest_format/legacy_ioi.py	35.59% <ø> (ø)
judge/highlight_code.py	46.15% <0.00%> (ø)
judge/management/commands/adduser.py	0.00% <ø> (ø)
judge/management/commands/makedmojmessages.py	0.00% <0.00%> (ø)
... and 20 more

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[leduythuccs]

Hi, could you share with your command to change all double quotes to single quotes?

I have a fork with huge changes :'(