Added unit tests and updated documentation

[aaronfultonnz]

I've added unit tests for:

• SQLite as the backend
• Updated the test for custom buckets to use SQLite (so the test does what it is supposed to) (test has moved down the file)
• Added unit test for the caching mixin #76

Update readme

• To use SQLite as the caching backend example as this is what is in the test
• Added a prominent note about rate limits being linked to sessions and when to use a custom backend

I've updated the pyproject.toml file but as I don't use poetry the poetry.lock file will be out of date now.

[JWCook]

Thanks for the updates!