adding static type checking to basic_binary_tree.py (#2293)

* adding static type checking to basic_binary_tree.py

* Add static type checking to functions with None return type

* Applying code review comments

* Added missing import statement

* fix spaciing

* "cleaned up depth_of_tree"

* Add doctests and then streamline display() and is_full_binary_tree()

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