Use f-strings instead of str.format()

[bakhtos]

Closes #427 -All instances of using str.format() for printouts/error messages are changed to f-strings; -Certain f-strings utilize the property that putting '=' sign after expression prints first the expression and then the result, to avoid duplicate typing of things like f"len(list) = {len(list)}", which is now simply f"{len(list) = }". -requirements.txt now lists the minimal version of Python (3.6) to support f-strings.

[aarmey]

@bakhtos I enabled tests on here so you can find remaining syntax issues. These all look like pretty simple fixes.

[JeanKossaifi]

@bakhtos are you still working on this? It would be great to merge it for the next release!