requirements.txt: update to urllib3 2.6.3

[pjonsson]

This fixes security issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.60%. Comparing base (ec031a8) to head (ef36703).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #267   +/-   ##
=======================================
  Coverage   38.60%   38.60%           
=======================================
  Files           7        7           
  Lines         891      891           
  Branches       99       99           
=======================================
  Hits          344      344           
  Misses        508      508           
  Partials       39       39           

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