modify io/excel.py and relevant docs (io.rst) to use sheet_name for read_excel
but allow sheetname to still be used for backwards compatibility.  add test_excel
to verify that sheet_name and sheetname args produce the same result.

added @deprecate_kwarg to read_excel as arg changes from sheetname to sheet_name.
moved test comments into function, add assert_produces_warning.

remove manual arg change, use @deprecate_kwarg to read_excel
as arg changes from sheetname to sheet_name.

shorten lines under 79 char.

update whats new 0.21.0 Deprecations section noting sheetname deprecated
in favor of sheet_name.
add sheetname deprecation in read_excel() docstring.