CLN Replace old string formatting syntax with f-strings #29547: 2 files w…
Hello, i'm new to github / opensource. Let me know if I'm doing things correctly

pandas/_libs/tslibs/timedeltas.pyx
pandas/_libs/tslibs/timestamps.pyx


Replace old string formatting syntax with f-strings #29547

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