Django upgrade

[justinrporter]

Now that we have python 3, upgrade django!

[codecov-io]

Codecov Report

Merging #263 into master will decrease coverage by 0.00%.
The diff coverage is 99.44%.

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
- Coverage   95.14%   95.13%   -0.01%     
==========================================
  Files         105       81      -24     
  Lines        4943     4793     -150     
==========================================
- Hits         4703     4560     -143     
+ Misses        240      233       -7     

Impacted Files	Coverage Δ
api/serializers.py	95.65% <ø> (ø)
appointment/forms.py	100.00% <ø> (ø)
audit/migrations/0001_initial.py	100.00% <ø> (ø)
followup/forms.py	95.08% <ø> (-0.16%)	⬇️
osler/base_settings.py	100.00% <ø> (ø)
pttrack/forms.py	100.00% <ø> (ø)
pttrack/test_validators.py	100.00% <ø> (ø)
referral/forms.py	92.00% <ø> (-0.16%)	⬇️
referral/migrations/0001_initial.py	100.00% <ø> (ø)
followup/models.py	91.57% <87.50%> (ø)
... and 48 more

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