CLN: GH29547 change string formatting with f-strings (6 files changed)
Replace old string formatting syntax with f-strings #29547

- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Changed 6 files:
- pandas/tests/io/parser/test_header.py
-  pandas/tests/io/test_sql.py
- pandas/tests/io/test_html.py
- pandas/tests/reductions/test_reductions.py
- pandas/tests/reshape/test_melt.py
- pandas/tests/scalar/timedelta/test_timedelta.py


thanks @MatteoFelici 