Add pyupgrade

[benjaoming]

Just a draft... overlapping with some changes in #1337 - will re-run once that's merged and released.

Thanks for the tip @valberg

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 79.52%. Comparing base (8e280fd) to head (742ecee).

Files	Patch %	Lines
src/wiki/decorators.py	0.00%	1 Missing ⚠️
src/wiki/plugins/attachments/models.py	0.00%	1 Missing ⚠️
...rc/wiki/plugins/editsection/markdown_extensions.py	0.00%	1 Missing ⚠️
src/wiki/plugins/images/models.py	0.00%	1 Missing ⚠️
src/wiki/plugins/macros/mdx/toc.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1338      +/-   ##
==========================================
- Coverage   79.52%   79.52%   -0.01%     
==========================================
  Files         114      114              
  Lines        4738     4737       -1     
  Branches      638      637       -1     
==========================================
- Hits         3768     3767       -1     
  Misses        759      759              
  Partials      211      211              

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