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_dynamodb2/test_dynamodb_table_with_range_key.py

@@ -8,7 +8,7 @@ from boto3.dynamodb.conditions import Key
 from botocore.exceptions import ClientError
 import sure  # noqa
 from freezegun import freeze_time
-from nose.tools import assert_raises
+import pytest
 
 from moto import mock_dynamodb2, mock_dynamodb2_deprecated
 from boto.exception import JSONResponseError
@@ -1353,7 +1353,7 @@ def test_update_item_with_expression():
 
 
 def assert_failure_due_to_key_not_in_schema(func, **kwargs):
-    with assert_raises(ClientError) as ex:
+    with pytest.raises(ClientError) as ex:
         func(**kwargs)
     ex.exception.response["Error"]["Code"].should.equal("ValidationException")
     ex.exception.response["Error"]["Message"].should.equal(