Convert old-style strings to python f-strings

[mkavulich]

Description

Since Python version 3.6, a much simpler method of handling variable substitutions in strings (Formatted string literals, or "F-strings" has been introduced. Since we have deprecated python 2, and all modern machines should support at least this version of python, we should convert all strings in the framework to use F-strings where appropriate.

Solution

Convert all old-format strings to f-string format.

Alternatives (optional)

Keep old-format strings, but this is undesirable. Plus there are already f-strings in the framework, so we already don't have compatibility with older Python versions.

[mkavulich]

Duplicate of #501