fix pygraphviz installation issue

[edublancas]

See #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)

[edublancas]

pygraphviz 1.8 dropped support for python 3.7

Solution:

pip install 'pygraphviz<1.8'

or:

conda install 'pygraphviz<1.8' -c conda-forge"