feat: setup cython example

[aryarm]

Description

This PR adds an example of some C code to our python package. It follows the template in https://github.com/BrianPugh/python-template?tab=readme-ov-file#cython

The C code defines a really simple divide() function which can be used to divide two numbers. You can test it out like this:

conda run -n panct poetry run python -c 'from panct._c_extension import divide; print(divide(6,3))'

or

conda activate panct
poetry shell
python
from panct._c_extension import divide
divide(6,3)

Note: every time a change is made to our C code, we will need to rerun poetry install to recompile

Still to do

• How do we want to include the gbz-base source code in the repo? Options: git submodule or install as setup step
• Do we want to automatically skip compilation of the cython module if it doesn't work? Or should we just fail/error explicitly? The former could be helpful if we want to implement some kind of slower alternative
• fix nox and GitHub actions for testing to allow them to compile wheels
• add CIBUILDWHEEL to GitHub actions build wheels on lots of platforms
• add gbz-base to _c_src? for rust: we need indexing and subsetting

[aryarm]

TODO: delete this file before merging