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_awslambda/test_awslambda_cloudformation.py

@@ -4,7 +4,7 @@ import sure  # noqa
 import zipfile
 from botocore.exceptions import ClientError
 from moto import mock_cloudformation, mock_iam, mock_lambda, mock_s3, mock_sqs
-from nose.tools import assert_raises
+import pytest
 from string import Template
 from uuid import uuid4
 
@@ -109,7 +109,7 @@ def test_lambda_can_be_deleted_by_cloudformation():
     # Delete Stack
     cf.delete_stack(StackName=stack["StackId"])
     # Verify function was deleted
-    with assert_raises(ClientError) as e:
+    with pytest.raises(ClientError) as e:
         lmbda.get_function(FunctionName=created_fn_name)
     e.exception.response["Error"]["Code"].should.equal("ResourceNotFoundException")