fix(google-auth): add cryptography bound for Python 3.14

[ohmayr]

Summary of Changes

Added "cryptography >= 41.0.5; python_version >= '3.14'" in setup.py and cryptography==41.0.5 in testing/constraints-3.14.txt.

Rationale

In Python 3.14, the legacy setuptools / pkg_resources module was removed from the standard build environment.
Older cryptography versions (< 41.0, including google-auth's default lower bound 38.0.3) utilized pkg_resources in their build hooks. When building from source (sdist) or installing on custom runtimes (such as Alpine Linux, ARM64, or non-manylinux environments), cryptography 38.0.3 fails with a ModuleNotFoundError: No module named 'pkg_resources' build error.
PyCA (Python Cryptography Authority) removed pkg_resources and updated setuptools-rust build hooks in cryptography 41.0.5+.
Setting "cryptography >= 41.0.5; python_version >= '3.14'" ensures:

1. Python 3.10–3.13: google-auth continues to support cryptography >= 38.0.3 (zero breaking changes for existing users).
2. Python 3.14+: pip automatically selects cryptography >= 41.0.5 so installations and builds succeed cleanly across all platforms and architectures.

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How to Reproduce

To reproduce the build failure on Python 3.14 without this fix:

1. In a Python 3.14 environment:

   pyenv activate py314
   pip install --no-binary :all: "cryptography==38.0.3"

[gemini-code-assist]

Code Review

This pull request updates the cryptography dependency requirements in setup.py to use version 41.0.5 or higher for Python 3.14 and above, and introduces a new constraints file constraints-3.14.txt for Python 3.14 testing. Feedback on the changes highlights that setuptools is included in the new constraints file but is missing from the primary dependency specification in setup.py, which violates repository guidelines.

[gemini-code-assist]

According to the repository's general rules, libraries listed in constraints files must be present in the project's primary dependency specification (e.g., setup.py). setuptools is not listed in the primary DEPENDENCIES of setup.py. Please verify if setuptools should be removed from this constraints file or if it needs to be added to the primary dependency specification.

References

1. When reviewing dependency changes, verify that libraries listed in constraints files are present in the project's primary dependency specification (e.g., setup.py) by checking the entire file, as they may be located outside the current diff window.