Fix byte handling to work for both Py2 and Py3

wrappers/python/simtk/openmm/app/dcdfile.py

@@ -73,7 +73,7 @@ class DCDFile(object):
         header = struct.pack('<i4c9if', 84, b'C', b'O', b'R', b'D', 0, firstStep, interval, 0, 0, 0, 0, 0, 0, dt)
         header += struct.pack('<13i', boxFlag, 0, 0, 0, 0, 0, 0, 0, 0, 24, 84, 164, 2)
         header += struct.pack('<80s', b'Created by OpenMM')
-        header += struct.pack('<80s', b'Created '+bytes(time.asctime(time.localtime(time.time())), 'ascii'))
+        header += struct.pack('<80s', b'Created '+time.asctime(time.localtime(time.time())).encode('ascii'))
         header += struct.pack('<4i', 164, 4, len(list(topology.atoms())), 4)
         file.write(header)