Replace python-jose with google alternative

[Jipje]

Closes #565

For more context to why python-jose should be replaced, please reference #565

This solution is a modified version of another repository of GoogleCloudPlatform: https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/iap/validate_jwt.py. Although the code was modified as to minimize the amount of changes necessary in this repository.

The certs() method and CERTS global have been replaced by a certs_url parameter in the verify_token method. This did not change behaviour in the tests.

[snippet-bot[bot]]

Here is the summary of changes.

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- [authenticating-users/main.py:25](https://github.com/GoogleCloudPlatform/getting-started-python/blob/d9da8db203f7729f5da28b57be66b69084955bde/authenticating-users/main.py#L25), tag `getting_started_auth_certs`

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[iennae]

@Jipje thanks for raising this and investigating a solution

/gcbrun

[Jipje]

Cheers @iennae, happy to help out. I am unable to view the details of the kokoro build so am unsure as to why it fails.

Is it anything to do with the adjusted # [START getting_started_auth_certs] comments?

[Jipje]

Bumping this @iennae