Integrate three different task runners with CI

[danielballan]

• make example for @rossbar ("Be accessible to current sphinx users!") ←make build is run by tox
• tox example for @bsipocz ("Test multiple versions!")
• pixi example for @danielballan ("Pixi is the future!")

Any given project should probably pick one! But this repo will demonstrate how to use all three, and it will test all three on CI.

[danielballan]

The pixi job is working (except on Windows):

Run pixi run test
✨ Pixi task (test in default): ./test.sh
loop in tutorials/executable/basics.md
[jupytext] Reading tutorials/executable/basics.md in format md
[jupytext] Writing tutorials/executable/basics.ipynb
loop in tutorials/static/static.md
loop in tutorials/matplotlib/interactive_mpl.md
[jupytext] Reading tutorials/matplotlib/interactive_mpl.md in format md
[jupytext] Writing tutorials/matplotlib/interactive_mpl.ipynb
loop in tutorials/matplotlib/static_mpl.md
[jupytext] Reading tutorials/matplotlib/static_mpl.md in format md
[jupytext] Writing tutorials/matplotlib/static_mpl.ipynb
============================= test session starts ==============================
platform linux -- Python 3.13.3, pytest-8.3.5, pluggy-1.5.0
rootdir: /home/runner/work/executable-tutorials/executable-tutorials
configfile: pytest.ini
plugins: anyio-[4](https://github.com/scientific-python/executable-tutorials/actions/runs/15002801972/job/42153879034#step:4:5).9.0, nbval-0.11.0
collected 12 items

tutorials/executable/basics.ipynb ....                                   [ 33%]
tutorials/matplotlib/interactive_mpl.ipynb .....                         [ 7[5](https://github.com/scientific-python/executable-tutorials/actions/runs/15002801972/job/42153879034#step:4:6)%]
tutorials/matplotlib/static_mpl.ipynb ...                                [100%]

============================= 12 passed in 11.93s ==============================
removed 'tutorials/executable/basics.ipynb'
removed 'tutorials/matplotlib/interactive_mpl.ipynb'
removed 'tutorials/matplotlib/static_mpl.ipynb'