Updated singly_linked_list (#2477)

* Updated singly_linked_list

* fixup! Format Python code with psf/black push

* undo __repr__

* updating DIRECTORY.md

* UNTESTED CHANGES: Add an .__iter__() method.

This will break tests, etc.

* fixup! Format Python code with psf/black push

* len(tuple(iter(self)))

* fixed __repr__()

* Update data_structures/linked_list/singly_linked_list.py

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

* Update data_structures/linked_list/singly_linked_list.py

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

* Update data_structures/linked_list/singly_linked_list.py

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

* Update data_structures/linked_list/singly_linked_list.py

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

* Update data_structures/linked_list/singly_linked_list.py

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

* Update data_structures/linked_list/singly_linked_list.py

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

* Update data_structures/linked_list/singly_linked_list.py

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

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