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_ec2/test_launch_templates.py

@@ -1,7 +1,7 @@
 import boto3
 import sure  # noqa
 
-from nose.tools import assert_raises
+import pytest
 from botocore.client import ClientError
 
 from moto import mock_ec2
@@ -30,7 +30,7 @@ def test_launch_template_create():
     lt["DefaultVersionNumber"].should.equal(1)
     lt["LatestVersionNumber"].should.equal(1)
 
-    with assert_raises(ClientError) as ex:
+    with pytest.raises(ClientError) as ex:
         cli.create_launch_template(
             LaunchTemplateName="test-template",
             LaunchTemplateData={