Fixes to test framework and GitHub workflow

Following suggestions from code review, this commit avoid re-using the working
directory from different versions of the library.

After many tries, I think I finally manage to find the right combination of
tweaks to make pytest, tox, and GitHub actions to work.

This also adds caching of the librocksdb compilation, which is a very slow
step, and restricts building of wheels and other artifacts to run only when
pushing to main.

Remove members that do not exist in rocksdb. Update configuration

Remove unnecessary delete

Revert deleting Close method call

While building the docker image pass the arguments `--build-arg
"host_uid=$(id -u)" --build-arg "host_gid=$(id -g)"` so that you can
use the outputs of the compilation outside

Update Docker ubuntu version and add Cython

Add dependency to Cython in pyproject.toml file

Update tox configuration for unit test

Remove deprecated setup_requires

Update README, set myself as current author/maintainer, and move most of setup.py to the configuration file.