Swap to Ruff

[jarekwg]

Replaces isort & flake8 with ruff.
Introduces a bunch of additional linting rules.
Removes setup.cfg

Notable changes:

• Providing a non-datetime object to oauth_expires_at now raises ValueError explicitly rather than using a plain assert.
• Bumped minimum python version to 3.10 to take advantage of some slightly nicer type checking.

Closes #90