CVXPY not available in Python 3.12

[stavros11]

@renatomello for the time being, I dropped the dependency on CVXPY, since it's not even possible to install on Linux without compiling the qdldl wheel with CMake (required by the OSQP solver).

However, I did not touch anything in the diamond_norm code, and I'm just skipping the related test. If you happen to have cvxpy installed, then diamond_norm will just work as before.

It seems like there are other people actively worried about these wheels, and the CVXPY team is aware. As soon as they will solve, I will reintegrate the dependency.
However, it is just a missing dependency declaration. Of course, it's not the best, but I believe it is better than requiring compilation to run qibo tests (or to install a qibo extra).

Originally posted by @alecandido in #1231 (comment)

The related test should also be unskipped.