Build: Allow building without cython installed (based on pre generated mpfr.c)

[LucasMahieu]

In case cython is not installed on the system, fallback on building the wheel with the mpfr.c.

This file was generated and added to the git (instead of being re generated thanks to cython).

This commits helps to express dependencies of a package, on bigfloat, from github repo. Without the guarantie that Cython is installed on the systeme.

[LucasMahieu]

This PR goals is to be able to depend on bigfloat and able to be able to get bigfloat installed (thanks to pip, pipenv, etc) in all cases :
If cython < 3 is installed
if cython > 3 is installed
if no cython is installed at all

The issue I was facing is that :

• I create a virtual env (empty
• I describe my 1st level dependencies (ex. req.txt, Pipefile)
• pip install ....
• But since my virtualenv is empty, pip can try to install bigfloat without having cython installed.
• Even if I add cython to my req.txt / Pipefile, the order of by dependencies is never guarantiee. So, it may lead to try to install bigfloat before having cython installed.

This commit solve this issue

EDIT: Note that since there is no 0.5.0 pypi package version available, we need to pip install from git directly : pip install git+https....