Pinned Requirement versions, drop Python 3.6 Support

[pgierz]

A while ago, I pinned a bunch of the requirement versions to ensure reproducible installs. This causes conflicts with some other packages the user might install if they are not using virtual environments (they should be doing that anyway, but, we know how it is in reality).

I would recommend going for the risk of getting weird behaviour (a very low risk) and unpinning the versions. This would avoid more confusing errors of installing esm-tools in ${HOME}/.local/lib/python-<XYZ> next to other packages that might already be there.

Also, we should drop Python 3.6, it's ancient.