Adhere to PEP484 for type hints

PEP484 introduces type hints in Python3, but also provides a suggested documentation syntax for packages that need Python 2 compatibility https://www.python.org/dev/peps/pep-0484/#suggested-syntax-for-python-2-7-and-straddling-code . It would be nice if PyRDF adhered to that scheme for all its functions and methods