Drop Django < 3.2, modernize the code

[matthiask]

Add pyupgrade and django-upgrade to automatically take advantage of new
features of Python and Django.

[codecov]

Codecov Report

Merging #1552 (d3289f8) into main (1964826) will decrease coverage by 0.55%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main    #1552      +/-   ##
==========================================
- Coverage   85.87%   85.31%   -0.56%     
==========================================
  Files          36       36              
  Lines        1883     1784      -99     
  Branches      310      298      -12     
==========================================
- Hits         1617     1522      -95     
+ Misses        187      184       -3     
+ Partials       79       78       -1     

Impacted Files	Coverage Δ
debug_toolbar/__init__.py	100.00% <ø> (+28.57%)	⬆️
debug_toolbar/panels/profiling.py	85.41% <0.00%> (-3.13%)	⬇️
debug_toolbar/panels/sql/tracking.py	86.32% <ø> (-1.92%)	⬇️
debug_toolbar/panels/templates/views.py	61.90% <0.00%> (-0.89%)	⬇️
debug_toolbar/panels/sql/views.py	81.25% <33.33%> (-9.80%)	⬇️
debug_toolbar/management/commands/debugsqlshell.py	89.47% <50.00%> (-10.53%)	⬇️
debug_toolbar/utils.py	76.47% <50.00%> (ø)
debug_toolbar/panels/cache.py	76.11% <60.00%> (+3.55%)	⬆️
debug_toolbar/panels/settings.py	100.00% <100.00%> (+12.50%)	⬆️
debug_toolbar/panels/signals.py	86.04% <100.00%> (-0.32%)	⬇️
... and 27 more

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[matthiask]

Oh damn, I reverted the push. Sorry.