ENH - Modify Dataframe.select_dtypes to accept scalar values
This commit relates to GH16855. It allows the Dataframe.select_dtypes
function to accept scalar values as well as list-like values. As such,
it should maintain backwards compatibility.

 - [x ] closes #16855
 - [x ] tests added / passed
 - [x ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [x ] whatsnew entry
