Add pydot test dependency
Add pydot test dependency

We need to use pydot rather than pydot3 because
of a broken version comparison in networkx, where it rejects
the lower version number of pydot3.

Without this patch the taskflow unit tests which use dot for
rendering graphics are no longer runnable.

Change-Id: I676e4d76627b3ecd0e4a9b896b2b93d652ebb25d

openstack/taskflow,openstack/taskflow,jimbobhickville/taskflow,jimbobhickville/taskflow