Blifparser

* fix dependencies: removed unused dependencies, made requirements.txt equal to setup.cfg dependencies
* added more comments, types and unit tests
* fix "Replacement index 1 out of range" error caused by missing string format parameter
* check before accessing variables that might be None and lists that might be empty

* Added get_graph() method to the BlifParser class: generates and returns an object containing a networkx graph with inputs, outputs, latches, subckts and boolean functions connected together.
> This object also contains statistics which can be used when exporting the graph to an image file to roughly determine the dimensions of the image.

* Dropped python < 3.7 support.