Replace `python-jose` with `pyjwt`

[lewisdoesstuff]

As python-jose seems to be unmaintained and has multiple vulnerabilities raised against it, I've replaced this with pyjwt.

The implementation is like-for-like, as pyjwt seems to implement most of the methods used from python-jose identically.

Updated unit test mock paths to new pyjwt locations.

Updated requirements.txt to include pyjwt

Haven't been able to run integration tests as I'm not entirely sure how I get an ID token via Postman, but that should run in CI.

Also didn't bump the version, but let me know and I'll update it.

Would resolve #54