Trim down dependencies, and only pin down minimal version

[NicolasHug]

Related to #65

There seems to be a lot of dependencies in requirements.txt that are actually not needed. For example scikit-learn, numba, joblib, threadpoolctl, six, etc. are not used and could be removed. There are probably a bunch of others, I didn't check them all.

Also, all dependencies are pinned-down to a specific version, e.g. Pillow==8.2.0. Unless there are very specific reasons to do so, it is preferable to only pin down the minimal version e.g. Pillow>=8.2.0 to allow for more flexibility in terms of environment setup.