py3 compat: fix base64_(en|de)code, int_to_bytes / bytes_to_int
py3 compat: fix base64_(en|de)code, int_to_bytes / bytes_to_int

also be more precise in docstrings about datatypes

mitsuhiko/itsdangerous,rootcanal/itsdangerous,mitsuhiko/itsdangerous,pallets/itsdangerous