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_swf/models/test_domain.py

@@ -4,9 +4,6 @@ import sure  # noqa
 from moto.swf.exceptions import SWFUnknownResourceFault
 from moto.swf.models import Domain
 
-# Ensure 'assert_raises' context manager support for Python 2.6
-import tests.backport_assert_raises  # noqa
-
 # Fake WorkflowExecution for tests purposes
 WorkflowExecution = namedtuple(
     "WorkflowExecution", ["workflow_id", "run_id", "execution_status", "open"]

tests/test_swf/responses/test_workflow_executions.py

@@ -4,9 +4,6 @@ from datetime import datetime, timedelta
 
 import sure  # noqa
 
-# Ensure 'assert_raises' context manager support for Python 2.6
-import tests.backport_assert_raises  # noqa
-
 from moto import mock_swf_deprecated
 from moto.core.utils import unix_time