Bump to version 2.2

[tim-schilling]

• Removed support for end of life Django 2.0 and 2.1.
• Added support for Python 3.8.
• Add locals() option for sql panel.
• Added support for Django 3.0

[codecov]

Codecov Report

Merging #1234 into master will increase coverage by 0.76%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
+ Coverage   85.72%   86.49%   +0.76%     
==========================================
  Files          25       25              
  Lines        1436     1436              
  Branches      206      206              
==========================================
+ Hits         1231     1242      +11     
+ Misses        150      141       -9     
+ Partials       55       53       -2

Impacted Files	Coverage Δ
debug_toolbar/panels/sql/tracking.py	88.88% <0%> (+1.85%)	⬆️
debug_toolbar/management/commands/debugsqlshell.py	100% <0%> (+10.52%)	⬆️
debug_toolbar/panels/sql/views.py	90.62% <0%> (+10.93%)	⬆️

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