Add urllib3 LICENSE.txt file

[hmstepanek]

Overview

The LICENSE.txt file was accidentally removed in a previous PR that upgraded urllib3.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.21%. Comparing base (38c2ba4) to head (a465475).
Report is 121 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1237      +/-   ##
==========================================
- Coverage   81.23%   81.21%   -0.02%     
==========================================
  Files         198      198              
  Lines       21675    21675              
  Branches     3434     3434              
==========================================
- Hits        17607    17604       -3     
  Misses       2937     2937              
- Partials     1131     1134       +3     

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