Detect when f-strings don't use any interpolation variables

[PCManticore]

Is your feature request related to a problem? Please describe

There is no error if a string is marked as an f-string but does not use any interpolation. e.g. f"some message" is superfluously marked as an f-string since it does not have any interpolated variables.

Describe the solution you'd like

We should emit a warning when we encounter f-strings without interpolation variables.

Additional context

Slightly related to #2507.