The Cython language makes writing C extensions for the Python language
as easy as Python itself.
Cython is a source code translator based on Pyrex, but supports more
cutting edge functionality and optimizations.

The Cython language is a superset of the Python language (almost all
Python code is also valid Cython code), but Cython additionally supports
optional static typing to natively call C functions, operate with C++
classes and declare fast C types on variables and class attributes.
This allows the compiler to generate very efficient C code from Cython
code.

This makes Cython the ideal language for writing glue code for external
C/C++ libraries, and for fast C modules that speed up the execution of
Python code.

You have to add those lines in your SlackBuild to make use of this
newer cython:
PYVER=$(python3 -c 'import sys; print("%d.%d" % sys.version_info[:2])')
export PYTHONPATH="/opt/python$PYVER/site-packages:/opt/cython/python$PYVER/site-packages"
export PATH="/opt/cython/bin:$PATH"
