Strongly connected components (#2114)

* Implement strongly connected components for graph algorithms

* fixup! Format Python code with psf/black push

* Delete trailing whitespace

* updating DIRECTORY.md

* Add doctests and typehints

* Remove unnecessary comments, change variable names

* fixup! Format Python code with psf/black push

* Change undefined variable's name

* Apply suggestions from code review

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>