Fix Issue 34748 - read in datetime as MultiIndex for column headers
- [x] closes #34748
- [x] tests added / passed
  - `pandas/tests/io/excel/test_readers.py`, method `test_read_datetime_multiindex`
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
