Replace string.format() with fstrings

[obulat]

Python 3.6 introduced a new way of formatting strings using so-called f-strings. This made formatted strings more readable and concize.

Instead of writing

who = "world"
print("Hello {who}".format(who=who))

we can write

who = "world"
print(f"Hello {who}")

This PR consistently replaces string.format with f-strings.

Signed-off-by: Olga Bulat [email protected]

[zackkrida]

I like that you also switched simple cases like VAR_NAME + 'test-123' to f'{VAR_NAME}test-123'; the consistency is appreciated.

[dhruvkb]

This is so much better, thank you!