Enable pyupgrade Ruff rules

[janluke]

Enable pyupgrade Ruff rules:

• UP006 collection annotations
• UP007 unions
• UP031 percent formatting
• UP032 f-strings
• UP035 deprecated imports
• UP037 quoted annotations
• UP045 optionals

A full --select UP audit now reports no findings.

Closes #226.

[codecov]

Codecov Report

❌ Patch coverage is 97.82609% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.71%. Comparing base (f3630fd) to head (b6e0ae0).

Files with missing lines	Patch %	Lines
src/cloup/_sections.py	85.71%	1 Missing ⚠️
src/cloup/constraints/conditions.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
+ Coverage   96.68%   96.71%   +0.02%     
==========================================
  Files          22       22              
  Lines        1540     1552      +12     
==========================================
+ Hits         1489     1501      +12     
  Misses         51       51              

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