cython software package provides C-Extensions for Python, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install cython
cython is installed in your system.
Make ensure the cython package were installed using the commands given below,
$ sudo dpkg-query -l | grep cython *
You will get with cython package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Python |
| Package | cython |
| Version | 0.23.4+git4-g7eed8d8-2 |
| Description | C-Extensions for Python |
| Sub Section | Python |