Patch release

Changes:

• Push the networkx dependency from 2.1 to 2.3 (2.1, 2.2, and 2.3 looked ok)
• Remove the local copy of _all_simple_paths_graph() that has now made it into networkx from 2.2 onward
• Update docs

This release should not change behavior or performance, but the _all_simple_paths_graph() function is a key component of cortexpy graph traversal, so please report any changes in performance.