Replace python-jose dependency

A vulnerability has been found in the `ecdsa` dependency which will not be patched in the `python-jose` package. `python-jose` [seems to be abandoned](https://github.com/mpdavis/python-jose/issues/332). Other people are also encountering these [security issues](https://github.com/mpdavis/python-jose/issues/341).

I suggest to update [authenticating-users/main.py](https://github.com/GoogleCloudPlatform/getting-started-python/blob/main/authenticating-users/main.py) to not use this insecure package. A commonly used alternative is PyJWT.