Calculate positive relative end index for last section in NumpydocTools

[wagnerpeer]

To fix the issue one had to calculate the correct, positive relative end index in NumpydocTools.get_list_key instead of using -1 as end index.

But because there are more issues with already existing numpydoc which gets "converted" to numpydoc I left the test tests/pyment_test_cases.FilesConversionTests.testCaseNoGenDocsAlreadyNumpydoc untouched and I also did not modify the tests/docs_already_numpydoc.py file as this would also obfuscate the current issue with using "Raises" as last section.
However I added a dedicated Python file named tests/issue_49.py and showed my fix working by adding the corresponding unit test in tests/test_issues.py

[wagnerpeer]

As this request got not merged yet, I am closing it to clean up my branches and send another request from another branch.

[dadadel]

@wagnerpeer sorry I didn't yet have a look to this PR.
I've just updated Pyment a new time.
Indeed do not hesitate to send another request.