Minor documentation and CI changes

This PR is a small agglomeration of changes:

Sets warnings to be failures on all actions, which should help capture problems in all feature combinations
Adds clippy to CI, which involses fixing or ignoring some warnings (which are now errors due to the above)
Fixes #215 and tweak the documentation action to also use all features
Bumps pyo3 to newest version, which compiles on stable

sparsemat / sprs