added Boruvka's MST algorithm (#2026)

* added Boruvka's MST algorithm

* Add files via upload

* fixup! Format Python code with psf/black push

* Updated Boruvka with doctest

* updating DIRECTORY.md

* Update minimum_spanning_tree_boruvka.py

* No blank line in doctest

* <BLANKLINE>

* Avoid mutable default values

https://docs.python-guide.org/writing/gotchas/

* Update minimum_spanning_tree_boruvka.py

* Avoid mutable default values

* fixup! Format Python code with psf/black push

* Update minimum_spanning_tree_boruvka.py

* Update minimum_spanning_tree_boruvka.py

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