Our OAuth library ought to be upgraded

[da1910]

Our existing OAuth library requests-oauthlib is irregularly maintained, does not support PKCE and is somewhat low-level. We ought to consider a more featured and maintained library.

I would suggest requests_auth, as it supports PKCE, automatic refresh token configuration and has a motivated maintainer. This would also allow us to remove the requirement for requests_ntlm, since it comes with NTLM authentication.