Windows dmypy CI flakes

[msullivan]

We've seen some failures of the daemon tests on windows CI:
https://ci.appveyor.com/project/gvanrossum/mypy/builds/20660260
https://ci.appveyor.com/project/gvanrossum/mypy/builds/20669303

The symptom is a timeout trying to connect while stopping the daemon when doing a
server-triggered restart in dmypy run

  Restarting: plugins changed
  error                   : Timed out waiting for connection. (diff)
  Daemon is stuck; consider C:\projects\mypy\mypy\dmypy.py kill (diff)
  == Return code: 1                             (diff)

@ethanhs

[msullivan]

https://ci.appveyor.com/project/gvanrossum/mypy/builds/21024302