Fix syntax warning raised by Python 3.8.
Fix syntax warning raised by Python 3.8.

flit/pyOCD,mesheven/pyOCD,pyocd/pyOCD,mesheven/pyOCD,mesheven/pyOCD,pyocd/pyOCD,mbedmicro/pyOCD,mbedmicro/pyOCD,mbedmicro/pyOCD,flit/pyOCD