Fix issue when using USE_TZ=False with MySQL

[maerteijn]

arrow always returns a tz aware datetime, and we don't want this when we explicitly configured Django with USE_TZ=False. This causes issues with MySQL (ERROR MySQL backend does not support timezone-aware datetimes when USE_TZ is False.)

This pull request makes the datetime object from arrow naive again when USE_TZ=False.

This fixes #350 and #352

[codecov-io]

Codecov Report

Merging #353 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #353      +/-   ##
=========================================
+ Coverage    90.4%   90.4%   +<.01%     
=========================================
  Files          43      43              
  Lines        2845    2846       +1     
=========================================
+ Hits         2572    2573       +1     
  Misses        273     273

Impacted Files	Coverage Δ
django_q/cluster.py	91.7% <100%> (+0.01%)	⬆️

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