Add url and typing hint for BFS (#2156)

* Add typing for bfs

* Add url for BFS

* rename the function

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update graphs/bfs.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Change the return value type of bfs

* change the function name.

change all instances of bfs() to breadth_first_search().

* change the function name in annotate

* Add one more blank line.

* Delete one blank line.

* Delete one blank line.

I've read the https://www.flake8rules.com/rules/W391.html, and still don't know how to do it. I've tried using 0 ,1,2 blank lines...

* Update graphs/bfs.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update graphs/bfs.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Rename bfs.py to breadth_first_search_2.py

Co-authored-by: Christian Clauss <cclauss@me.com>