Missing `f` prefix on f-strings

[code-review-doctor]

Some strings looks like they're meant to be f-strings but are missing the f prefix meaning variable interpolation won't happen.

pandas/pandas/tests/groupby/transform/test_transform.py

Line 1046 in f9762d8

result = result.rename(columns={c: "{c}_demeaned" for c in result.columns})

I found this issue automatically. I'm a bot. Beep Boop 🦊. See other issues I found in your repo here