This commit fix three test cases in test_index.py
* SeqTestCase.test_index
* SeqTestCase.test_wrappers
* ListTestCase.test_setdelitem
All these three cases' failures were caused by not supporting classes
that override __index__ operator when indexing sequence type.
This commit implement almost same routine with PR(#2807) which also did not
support classes which override __index__ operator in range indexing.
Signed-off-by: snowapril <sinjihng@gmail.com>