Change .format() strings into f"" strings

[Scottan]

Throughout the course, we should replace str.format() strings with formatted string literals ("f" strings).

https://realpython.com/python-f-strings/

They're clearer to read, quicker to write when giving the lesson and the Python recommended style to use. I'd also recommend replacing statements like print("abc", x, "def", y) with print(f"abc {x} def {y}"), although that is more work and less important.

We should probably add a box in the introduction about using f-strings and comparing them to str.format()