CVC Import Conditioned on Solver Flag

CVC is only imported if it is the selected solver. This allows users to use PyExZ3 without having CVC installed. I believe this fixes the issue we discussed on https://github.com/thomasjball/PyExZ3/pull/11.

To help prevent situations like this in the future I'm going to incorporate a Windows-based Z3-only environment in my test process. Please let me know if you see any other issues or incompatibilities.