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.

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.

This change:
* uses to setuptools Cython automatic extension build system.
* Add tox.ini to run tests and build docs into virtualenv
* Add .travis.yaml and Dockerfile to run tests in CI
* Change requirements to ensure:
  - Cython and setuptools are installed before we build the Cython
    extension
  - tests dependencies are not installed by default
  - doc dependencies are explicit
* Add missing lz4 library
* Allow to build the module with any librocksdb headers (no-rtti)

Closes #15