Update aniso8601 dependency to allow versions 4+. (#935)

* Update aniso8601 dependency to allow versions 4+.

* Upper bound to aniso8601 dependency version.

Author: crunk1

setup.py

@@ -88,7 +88,7 @@ def run_tests(self):
         "six>=1.10.0,<2",
         "graphql-core>=2.1,<3",
         "graphql-relay>=0.4.5,<1",
-        "aniso8601>=3,<4",
+        "aniso8601>=3,<=6.0.*",
     ],
     tests_require=tests_require,
     extras_require={