The blockdiag/seqdiag set of tools and their sphinx extensions are no
longer maintained. This hasn't been a huge issue until we started
running jobs on Python3.12 as we need to run an older version of Pillow
to support these tools and that needs special libs to build wheels on
python3.12.
Rather than continue to try and make old unmaintained tools work we
switch to graphviz which is maintained and has support built into
sphinx. This does require us to install graphviz as a system dep but
that seems like a reasonable tradeoff for using supported tooling.
The resulting graph specifications are also slightly more verbose.
Co-Authored-By: James E. Blair <jim@acmegating.com>
Change-Id: I2d1e4c3d648723402aae2d87fb3233f4418d5003
cb0b00ed87