graphviz needs to be in requirements.txt

[tmadlener]

podio-vis needs graphviz. If it is not there some tests will fail. We need to document this and also put graphviz into the requirements.txt file.

[m-fila]

As for the documentation it's already mentioned in the README

podio/README.md

Lines 75 to 78 in 1fc075d

	Some tools have additional dependencies that are not required for code generation or library use
	- `graphviz` is required for `podio-vis`
	- `tabulate` is required for `podio-dump`

[tmadlener]

Yes, but apparently people don't read that ;) This also ties in a bit with #474 if we want to do it "properly"