Relax cvxopt constraint on 1.2.1 to allow conda installation

[danielecc]

Right now requirements.txt imposes an exact cvxopt==1.2.1 version. This allows for a pip installation, but the conda package manager only has version 1.2.0 available, which makes the installation conflict and potentially fail.

Describe the solution you'd like
Relax the installation requirement to cvxopt>=1.2.0 or cvxopt==1.2.* to allow installation of DPPy in a conda environment without using pip for anything beside DPPy itself.

Describe alternatives you've considered
Other possibilities are:

• changing the constraint to cvxopt==1.2.0, and give up on the bugfixes of the point release
• open an issue asking continuum to update their package, can be done here
• install cvxopt from conda-forge, the community-maintained conda repo. However as of now the package is at version 1.2.3
• install DPPy and dependencies only using pip in a ad-hoc environment to avoid conflicts

Additional context
This was the only requirement I could not meet using conda. Note that for generating the docs sphinxcontrib-bibtex is also not present in the default conda repo, but can be installed through conda-forge. Overall, it would be nice to have an easy installation path through conda, and eventually to be able to install DPPy directly using conda-forge.