Fix bug in envvar source tests when run in Python 2.7

Due to an import error the envvar source tests would not run
in Python 2.7

tests/test_octodns_source_envvar.py

@@ -1,6 +1,12 @@
 from six import text_type
 from unittest import TestCase
-from unittest.mock import patch
+
+try:
+    # Python 3
+    from unittest.mock import patch
+except ImportError:
+    # Python 2
+    from mock import patch
 
 from octodns.source.envvar import EnvVarSource
 from octodns.source.envvar import EnvironmentVariableNotFoundException