Remove obsolete urllib3 upper bound

[sui294801854]

Summary

• remove the obsolete urllib3<2.1.0 cap from setuptools metadata
• align requirements.txt with the already-unbounded Poetry dependency declaration
• allow current patched urllib3 releases to resolve

Closes #21.

Validation

• built the wheel on Python 3.13
• installed it with urllib3==2.7.0 and the downstream constraint set
• python -m pip check
• python -m pytest -q (62 passed)
• pip-audit (No known vulnerabilities found)