Ensure setup.py compatiblity with recent Python versions.

[hatzel]

I do not know why the cvxopt version was pinned, but due to changes in configparser (a standard library module), the required version can no longer be installed with recent Python versions.

In my limited testing, I did not encounter any issues with this patch.

[maltem-za]

Thanks - cvxopt is a dependency of pygamma-agreement (not installed by default except for in the Docker image) and was pinned because we ran into some kind of compatibility issue. The Python version was pinned for a similar reason, although I can't recall now if it was also related to pygamma-agreement. We need to test everything with the latest versions and see if we can remove these restrictions (and possibly also remove cvxopt from setup.py). Will leave this open until we get a chance to do that.