* updating DIRECTORY.md
* Update sierpinski_triangle.py header doc
* Remove unused PROGNAME var in sierpinski_triangle.py
The PROGNAME var was used to print an image description in the reference
code that this implementation was taken from, but it's entirely unused
here
* Refactor triangle() function to not use list of vertices
Since the number of vertices is always fixed at 3, there's no need to
pass in the vertices as a list, and it's clearer to give the vertices
distinct names rather than index them from the list
* Refactor sierpinski_triangle.py to use tuples
Tuples make more sense than lists for storing coordinate pairs
* Flip if-statement condition in sierpinski_triangle.py to avoid nesting
* Add type hints to sierpinski_triangle.py
* Add doctests to sierpinski_triangle.py
* Fix return types in doctests
* Update fractals/sierpinski_triangle.py
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>
