fix(google-auth): add bounds for urllib3 and packaging dependencies

[ohmayr]

1. setup.py:
   • Added version bounds for urllib3 and packaging in urllib3_extra_require:

     urllib3_extra_require = [
         "urllib3 >= 1.26.15, < 3.0.0",
         "packaging >= 20.0",
     ]

   • Set requests >= 2.30.0, < 3.0.0 for Python 3.10+ compatibility.
   • Resolved and removed TODO comment for issue Removing py26 and pandas from default tox environments. #1739.
2. testing/constraints-3.10.txt:
   • Added requests==2.30.0, urllib3==1.26.15, and packaging==20.0 to lower-bound CI test constraints.

Fixes #15155 🦕

[gemini-code-assist]

Code Review

This pull request introduces version bounds for the urllib3 and packaging dependencies in setup.py, addressing a TODO item. Additionally, it pins urllib3 to 1.26.15 and packaging to 20.0 in the testing constraints file constraints-3.10.txt. There are no review comments, and I have no feedback to provide.