blockdiag, seqdiag and their associated sphinx extension are no longer
maintained. This wasn't an issue until the version of Pillow we need for
blockdiag/seqdiag stopped having wheels built for python3.12 and we
moved our default build env to python3.12. The good news is that sphinx
has a built in graphviz extension which should be well maintained that
we can switch to.
To make that switch we do need to install graphviz for the `dot` command
instead of libjpeg-dev since graphviz isn't a native python tool. But
this tradeoff seems reasonable since this software is maintained.
Change-Id: I60ba6235fcfc28508ce10bb60854c0dc62705e0c
eb8826b605