Get it working

wrappers/python/src/swig_doxygen/swig_lib/python/typemaps.i

@@ -156,3 +156,6 @@
   $3 = &tempC;
 }
 
+%typemap(out) std::string OpenMM::Context::createCheckpoint{
+    $result = PyBytes_FromString($1.c_str());
+}
\ No newline at end of file

wrappers/python/tests/TestBytes.py

+import unittest
+import simtk.openmm as mm
+
+
+class TestBytes(unittest.TestCase):
+    def test_createCheckpoint(self):
+        # check that the return value of createCheckpoint is of type bytes (non-unicode)
+        system = mm.System()
+        system.addParticle(1.0)
+        mm.Context(system, mm.VerletIntegrator(0))
+        chk = mm.Context(system, mm.VerletIntegrator(0)).createCheckpoint()
+        assert isinstance(chk, bytes)
+
+if __name__ == '__main__':
+    unittest.main()