Create merge_two_lists.py that implements merging of two sorted linked lists (#3874)

* Create merge_two_lists.py that implements merging of two sorted linked lists

* Update merge_two_lists.py

Fixed formatting errors

* Fixed trailing whitespace

* Change name of function to def __str__()

* updating DIRECTORY.md

* Imported classes from singly_linked_list.py

* Update merge_two_lists.py

* Update merge_two_lists.py

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>