FIX: fix path suffix condition in core/read.py

[je-cook]

In #620 the path suffix operation was updated to fix a warning in sphinx 8. The effect was to only check the last suffix instead of all suffixes as was previously achieved. For a filename my_example.ex.py the old value of path.endswith(suffix) would be true whereas Path.suffix only returns '.py' therefore will return false here:

MyST-NB/myst_nb/core/read.py

Line 71 in 83f8d1b

if Path(path).suffix == suffix:

I've added a test for this but I needed to do quite a bit of playing to get past some of the eccentricities of the test suite. let me know if I need to change anything

The reason for all of this on my end is to distinguish which files I want converted and not all python files in a given directory. For reference see where we do this

https://github.com/Fusion-Power-Plant-Framework/bluemira/blob/develop/documentation/source/conf.py#L130-L135