Update pyyaml requirement to >=5.1.
Update pyyaml requirement to >=5.1.

The 4.1 release series apparently had serious issues and may be
withdrawn soon. Plus it's good to get off a beta.

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