Improve DatetimeIndex.time performance
   xref #18058 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

  Speed up DatetimeIndex.time in a similar way to DatetimeIndex.date. Not sure whether the timezone information should be passed to `ints_to_pydatetime()` or not.