DEPR: deprecate non keyword arguments in read_excel
Follow-up to #27573.

Allows two non-keyword arguments, ``io`` and ``sheet_name``. I think ``sheet_name`` is quite often (e.g. Interactively) supplied without being a keyword argument and requiring it will just be needlessly annoying.

Also some clean-up in pandas/tests/io/excel.
Great PR @topper-123 