Use f-strings where sensible

[akx]

This PR replaces various string formatting operations with Python 3.6+ f-strings, for performance and brevity.

Guided by Flynt and Ruff's relevant lints, checked by hand.

[cclauss]

Do ruff EM rules object to some of these uses?

[akx]

@cclauss Not any more than they would with other formatting. I don't really agree with the EM rules because they make the code longer and more cumbersome to read. Henry's point is

If this is longer or more complex, the duplication can be quite confusing for a user unaccustomed to reading tracebacks.

and I would assume people who use MQTT should be accustomed to reading tracebacks.

[cclauss]

@akx git conflicts here.

[akx]

@cclauss Thanks, rebased.