Name(s):
PyPy, PyPy3, PyPy-STM

Homepage:
https://pypy.org/download.html

Why should it be included:
PyPy is a the main JIT python system, and is fairly heavily used by developers for that reason. PyPy and PyPy3 support the two language versions of Python, while PyPy STM is a highly optimized I/O version of PyPy.

How many users:
I don't have a feel for how large Solus is, but I've never met an experience Python developer who hasn't used it.

Source(s)/builds:
https://pypy.org/download.html#other-versions
https://bitbucket.org/pypy/pypy/src

Additional notes:
This can't be installed via pip or pip3 due to technical reasons. Most other Python interpreters can, including Cython, which you still have as a standalone package (though no other pip-able python packages are). You may want to develop some sort of rule regarding including or excluding libraries that are already available in a languages default package manager.