chore: add warning for rsa library

[daniel-sanche]

The rsa library is archived, and scheduled to be removed. google-auth already supports an alternate implementation, using cryptography. This PR adds a warning to users still relying on the old library, and adds rsa as an optional dependency to allow users to continue to opt-in to rsa during deprecation persion

After release, a follow-up version will remove rsa as a required dependency, leaving it only for opt-in users

[gemini-code-assist]

Summary of Changes

Hello @daniel-sanche, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request initiates the deprecation process for the rsa library within google-auth. It introduces a warning for users still relying on rsa and transitions rsa from a required dependency to an optional one, preparing for its eventual removal. This change guides users towards the cryptography library, which google-auth already supports, ensuring a smoother transition away from the archived rsa library.

Highlights

• Deprecation Warning for 'rsa' Library: A FutureWarning has been added to google/auth/crypt/_python_rsa.py to inform users that the rsa library is deprecated and will be removed in a future release, advising migration to cryptography.
• Optional 'rsa' Dependency: The rsa library has been transitioned to an optional dependency. Users who wish to continue using it during the deprecation period can now explicitly install it via google-auth[rsa].
• Warning Test Coverage: A new unit test has been added to tests/crypt/test__python_rsa.py to ensure that the FutureWarning is correctly raised when the _python_rsa module is imported.
• Noxfile Update: The noxfile.py has been updated to allow passing additional positional arguments to the pytest command during unit test execution, enhancing testing flexibility.

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[gemini-code-assist]

Code Review

This pull request introduces a FutureWarning to notify users about the upcoming deprecation and removal of the rsa library, which is being replaced by cryptography. To facilitate a smooth transition, an optional dependency google-auth[rsa] is added, allowing users to explicitly opt-in to continue using the rsa implementation during the deprecation period. The changes also include a new test to verify that the warning is correctly issued and a minor improvement to the noxfile.py for better testing flexibility. The changes are well-structured and effectively implement the first step of the deprecation plan.

[vchudnov-g]

Should this be a DeprecationWarning? I think that might be more appropriate if it doesn't introduce complications with existing tests, etc.

(not a blocker)

[daniel-sanche]

I was debating that too. I don't think the user vs developer divide described in the docs is super clear for me in this context.

Thinking again, maybe DeprecationWarning is a bit more accurate here. I made the change