NotImplementedError exception on Windows

[brwarner]

When running the latest code on master (efdfb70), running proof.py on Windows results in the following error.

Task exception was never retrieved
future: <Task finished coro=<Job.run() done, defined at proof.py:423> exception=NotImplementedError()>
Traceback (most recent call last):
  File "proof.py", line 431, in run
    await self.do_work()
  File "proof.py", line 445, in do_work
    process = await asyncio.create_subprocess_exec(self.command[0], *self.command[1:], stdout=fout, stderr=ferr, stdin=fin)
  File "C:\hostedtoolcache\windows\Python\3.7.9\x64\lib\asyncio\subprocess.py", line 217, in create_subprocess_exec
    stderr=stderr, **kwds)
  File "C:\hostedtoolcache\windows\Python\3.7.9\x64\lib\asyncio\base_events.py", line 1544, in subprocess_exec
    bufsize, **kwargs)
  File "C:\hostedtoolcache\windows\Python\3.7.9\x64\lib\asyncio\base_events.py", line 462, in _make_subprocess_transport
    raise NotImplementedError
NotImplementedError
Task exception was never retrieved

I did a little bit of Googling. Looks like some features of subprocess are not supported on Windows?

https://stackoverflow.com/questions/44633458/why-am-i-getting-notimplementederror-with-async-and-await-on-windows
fastapi/fastapi#964

It appears there might be some way to use a different back-end implementation or something that will work on Windows, but I don't know enough about python to really say.