Replaced .format{}/% with f-strings in core/tools/datetime.py
Modified to python3 format strings
ref: https://github.com/pandas-dev/pandas/issues/29547


- [x] contributes to fstring corrections in  #29547
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
~~whatsnew entry~~ (will not add now. I think the full set of changes in #29547 should get a single update entry rather than a piecemeal one)

suggest labels clean & style for this pull request, & part of the 1.0 milestone. thanks @JMBurley 