gmpy2 requires libmpc-dev & libmpfr-dev

[paddyroddy]

In my PR I neglected the fact that the gmpy2 library requires libmpc-dev & libmpfr-dev libraries #17 so either we need those in the setup.py or we should revert that PR

[jasonmcewen]

@mdavezac Just pinging you on this issue. We either need to add dependencies or revert to scipy.

[mdavezac]

It's been reverted in "main" now. I guess the best way to do it would be to require only scipy, but check if gmpy2 is installed and fallback to scipy otherwise. Users can then install gmpy2 independently if they feel the need.