use f-strings

[ikamensh]

I've used flynt to convert the source to use f-strings:

> flynt . -ll 110 

Execution time:                            1.278s
Files modified:                            19
Character count reduction:                 1400 (0.22%)

Per expression type:
No old style (`%`) expressions attempted.
`.format(...)` calls attempted:            169/170 (99.4%)
No concatenations attempted.
F-string expressions created:              158

Since unit tests only run for python 3.6+, I gathered this should be acceptable for the dependencies.

I've also done a manual change in a place where f-string wouldn't be created automatically:

"{0} is not a valid comparison operator: {0}".format(operator)
-> "{operator} is not a valid comparison operator."

[ikamensh]

Hello hello - any opinions on this?

[ikonst]

Maybe we should pre-commit along with pyupgrade to modernize this and other aspects of our codebase?