Switch to use oauthlib instead of oauth2

[liffiton]

What are the relevant tickets?

#78

What does this PR do?

This pull request drops the unmaintained oauth2 library in favor of oauthlib, which is maintained and appears to be more robust. In the process, this drops support for older versions of Python, now requiring 3.7+.

How should this be manually tested?

pip install -r requirements.txt
pytest

All tests pass. I've manually tested it with Python 3.7, 3.8, 3.9, 3.10, and 3.11.

Where should the reviewer start?

All substantial code changes are in pylti/common.py. Most large removals are removing workarounds for bugs in oauth2 that are now not needed.

I tried to stick as close to oauth2's implementation as possible. This includes not recording nonces to check for reuse, which is a security issue, but no worse than it was before. (And fwiw, while the LTI spec says that tool providers "should keep a record of nonces received," it also talks about an implementation detail "if you do not record nonces" as well.)

The rest is minor test cleanup, dependency updates, etc.

I'd be happy to make further commits for style or correctness.

[gitguardian]

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GitGuardian id	GitGuardian status	Secret	Commit	Filename
-	-	Generic Private Key	88ce21c	pylti/tests/data/certs/snakeoil.pem	View secret

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