add PKCE Oauth2 flow

[rgammans]

I've finally gotten around to cleaning this up and adding some documentation, tests etc.

Fixes: #316

[rgammans]

I've updated it to lint cleanly with flake8.

As to whether it's the best approach, it's hard to say, but there are a good number of places a user can customise it if necessary. And it's easy to use in the simple cases.

But without knowing more use cases it's difficult to write documentation with advice for them.

[rgammans]

Hmm. I'll probably need to run isort, but master isn't clean against isort 5.10.1 either.

[rgammans]

Ah, I misspoke about isort cleanliness it turns out there are some config updates