TST: Use Skipif Decorators in 'test_excel.py'

[WillAyd]

xref #19803

The skip_if_no* methods in test_excel.py could all be replaced with the skip_if_no decorator that was introduced in #18765

import pandas.util._test_decorators as td

@td.skip_if_no('xlwt')
def foo(self):
  ...

@td.skip_if_no('xlrd', min_version='0.9')
def bar(self):
  ...