Graph list patch (#4113)

* new implementation for adjacency list graph

* add example code for undirected graph

* reduce length to 88 columns max to fix build errors7

* fix pre commit issues

* replace print_list method with __str__

* return object in add_edge method to enable fluent syntax

* improve class docstring and include doctests

* add end of file line

* fix pre-commit issues

* remove __str__ method

* trigger build

* Update graph_list.py

* Update graph_list.py

Co-authored-by: gnc <[email protected]>
Co-authored-by: Christian Clauss <[email protected]>