Timedeltas: Understand µs
- [ ] closes #xxxx
- [X] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry


These are emitted by golangs `time.Duration` printing: https://github.com/golang/go/blob/36b815edd6cd23d5aabdb488c24db2033bbdeea2/src/time/time.go#L669