Upgrade to Django 3.2 (fixes #431)

[aptiko]

Checklist:

• [ ] All tests are passing
• [ ] I have added any necessary documentation and/or release notes
• [ ] I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• [ ] If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• [ ] I have removed all obsoleted code

[codecov[bot]]

Codecov Report

Merging #442 (16a043f) into master (4ed819b) will decrease coverage by 0.39%. The diff coverage is 63.15%.

@@            Coverage Diff             @@
##           master     #442      +/-   ##
==========================================
- Coverage   98.03%   97.63%   -0.40%     
==========================================
  Files          19       19              
  Lines        1473     1482       +9     
==========================================
+ Hits         1444     1447       +3     
- Misses         29       35       +6     

Impacted Files	Coverage Δ
enhydris/api/csv.py	100.00% <ø> (ø)
enhydris/models.py	98.28% <41.66%> (-1.29%)	:arrow_down:
enhydris/admin/station.py	98.74% <100.00%> (ø)
enhydris/views_common.py	93.63% <100.00%> (+0.17%)	:arrow_up:

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