Port test suite from nose to pytest.

This just eliminates all errors on the tests collection. Elimination of
failures is left to the next commit.

tests/test_core/test_moto_api.py

@@ -1,6 +1,6 @@
 from __future__ import unicode_literals
 import sure  # noqa
-from nose.tools import assert_raises
+import pytest
 import requests
 
 import boto3