%s changed to fstring, single quotes in f string changed to double qu…
https://github.com/pandas-dev/pandas/issues/29547#issuecomment-562164065
https://github.com/pandas-dev/pandas/issues/29547

- [x] pandas/_libs/tslibs/frequencies.pyx
- [x] pandas/_libs/tslibs/period.pyx
- [x] pandas/_libs/tslibs/strptime.pyx

changed %s to f strings and f ' '  to f " ".
str.format() remained unchanged due to:
https://github.com/pandas-dev/pandas/issues/29547#issuecomment-562227987 