text: Title: fix pygraphviz installation issue
  Body: See https://github.com/ploomber/ploomber/issues/538
  
  recent versions of pygraphviz only work with python 3.8 and higher, however, they didn't pin the dependency and raise an error instead. we should find what's the most recent version that works with python 3.7 and lower, and pin it. But do not pin it if running >=3.8 (there's a way to specify these rules in setup.py)