distributed: HLG serialization bug

What happened: Serialization fails with TypeError: can not serialize 'numpy.uint16' object when using master.

What you expected to happen: Serialization should succeed.

Minimal Complete Verifiable Example:

import dask.array as da
import numpy as np
from distributed.protocol.highlevelgraph import highlevelgraph_pack

def fn(x, dt):
    return x.astype(dt)

arr = da.blockwise(fn, "x", da.ones(1000), "x", np.uint16(0), None, dtype=np.uint16)

highlevelgraph_pack(arr.__dask_graph__(), None, None)

Anything else we need to know?: This happens for a number of numpy data types.

Environment:

• Dask version: 2021.02.0+3.ga14e47f
• Distributed version: 2021.02.0+1.g7d2a22f
• Python version: 3.7.6
• Operating System: PoP!_OS 18.04 LTS
• Install method (conda, pip, source): pip