boruvka.py: A few simplifications and f-strings (#4660)

* boruvka.py: A few simplifications and f-strings

Python f-strings simplify the code and [should speed up execution](https://www.scivision.dev/python-f-string-speed). 

@srkchowdary2000 Your review, please.

* updating DIRECTORY.md

* fixup! Streamline the test

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