Pin pillow<10 in CI (#922)

Since the recent release of Pillow 10.0.0 the docs CI job has started
failing due to an error in Pillow when trying to run the jupyter-execute
cell in the `.to_dot()` docstring. It looks like a bug that was
introduced in the new release which is being tracked in
python-pillow/Pillow#7259 where it's trying to return a jpeg
representation of the object from the RGBA data loaded from a PNG. Until
the issue is resolved upstream in pillow this commit just caps the
version we run in CI via the constraints file. While pillow is an
optional dependency and we could cap the version in the extras, this
issue isn't severe enough to warrant that, and the typical pillow usage,
especially via the rustworkx api (i.e. graphviz_draw() which returns a
PIL.Image object) will continue to work as expected with pillow 10.0.0
there isn't a reason to cap more broadly. This is just needed as a
workaround to unblock CI.

constraints.txt

@@ -1,2 +1,3 @@
 decorator==4.4.2
 importlib-metadata==4.13.0;python_version<'3.8'
+pillow<10.0.0