Rewrite tests_vprec_backends with python-gmp instead of python-bigfloat

[pablooliveira]

To remove a pip dependency.

[yohanchatelain]

I just discovered that the decimal package is part of the python standard library. That could be a good option to replace bigfloat without adding an extra dependency.

[pablooliveira]

Yes, in particular, bigfloat requires manual installation with the latest linux distributions which is a hassle. I agree that replacing it with decimal would be nice !

[titeup]

@yohanchatelain any updates? Do you plan to do it?

[yohanchatelain]

Hi @titeup, I did not work on it and actually it only offers arbitrary precision for Decimal representation so we'll have issue rounding numbers to binary precision. But we could reimplement what we need in pure C MPFR to get rid of bigfloat.

[yohanchatelain]

Done with #339.