fix(google-auth): raise rsa extra lower bound to 4.0

[ohmayr]

Summary of Changes

1. setup.py:
   • Updated rsa_extra_require from "rsa>=3.1.4,<5" to "rsa>=4.0.0,<5".
2. testing/constraints-3.10.txt:
   • Added rsa==4.0.0 to lower-bound test constraints.

---

Technical Rationale

Python 3.8+ Standard Library Removal (base64.decodestring):

• In Python 3.8, the deprecated base64.decodestring() function was completely removed from Python's standard library in favor of base64.b64decode().
• rsa < 4.0.0 (such as rsa 3.1.4 / 3.4.2) called base64.decodestring() internally in rsa/pem.py:92 during PEM certificate and key decoding.
• When running google-auth unit tests or loading PEM keys on supported Python versions (Python 3.10+), rsa==3.1.4 fails with:

  E   AttributeError: module 'base64' has no attribute 'decodestring'
  ../../../site-packages/rsa/pem.py:92: AttributeError
  
  Impact & Backward Compatibility
  

Non-breaking: rsa 4.0.0 was released in 2018. Virtually all active Python environments using google-auth[rsa] or google-auth[testing] already use rsa 4.x.
Primary Auth Flow Unaffected: google-auth uses cryptography as its primary required dependency for RSA/EC signing; rsa is only an optional extra (google-auth[rsa]).

[gemini-code-assist]

Code Review

This pull request updates the minimum version of the rsa dependency in packages/google-auth/setup.py from 3.1.4 to 4.8. There are no review comments, and I have no additional feedback to provide.