refactor: use f-strings instead of str format

[CarsonHam]

Formatted string literals, or f-strings, were a new feature introduced in Python 3.7, and according to several articles, like this one, they are over 3x faster than using the str format method.

The contributing guidelines for this project state that the use of f-strings is "encouraged", and it is an easy fix to go back and change all occurrences of str format to f-strings.

I would be happy to make the update myself. I will submit a pull request as soon as I implement the changes.

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