py-natsort: updated to 6.0.0

6.0.0:
Drop support for Python 2.6 and 3.3
Remove deprecated APIs (kwargs number_type, signed, exp, as_path, py3_safe; enums ns.TYPESAFE, ns.DIGIT, ns.VERSION; functions versorted, index_versorted)
Remove pipenv as a dependency for building
Simply Travis-CI configuration
Fix README rendering in PyPI

(adam)