RF: make cvxopt optional for tests

Protect against errorts in tests when cvxopt not installed.
BF: fix funky error using __import__ on cvxopt

Very strange error importing cvxopt with python3 and __import__; maybe
this is because I was using an half-documented form of the function
(passing an empty string in `fromlist`).

Don't pass empty string when it's not needed in optpkg
Merge pull request #363 from matthew-brett/cvxopt-optional

RF: make cvxopt optional for tests