BUG: read_excel return empty dataframe when using usecols
- [x] closes #18273
- [x] tests added / passed
- [x] passes git diff master --name-only -- "*.py" | grep "pandas/" | xargs -r flake8
- [x] whatsnew entry

As mentioned read_excel returns an empty DataFrame when usecols argument is a list of strings.
Now lists of strings are correctly interpreted by read_excel function.