Ruff rule suggestion: warn for useless f-strings

[Datamine]

Description

Occasionally I have seen f-strings used like this:

my_variable = "foo bar"
print(f"{my_variable}")

if f"{my_variable}" == "baz":
    return

In these instances, the f-string usage is improper because:

• The f-string contains only this variable, and nothing else. Using the f-string syntax is superfluous.
• If the user wanted to use a non-string variable as a string, then explicitly casting it using str(my_variable) would be more Pythonic.

I haven't seen a RUF rule to this effect, though I've seen a few other f-string rules. I think this would be a fine addition.

[MichaReiser]

Thanks for this rule suggestion.

As you pointed out: Whether the use of my_variable is unnecessary depends on if my_variable is already a string. Ruff's capability to detect this right now are very limited and I worry that the rule would have too many false-negatives (it couldn't flag an f-string like that if it can't determine the type of my_variable.

Related #16103