Windows regression: tests get stuck in CI after first one

[webknjaz]

It looks like smth in #80 breaks the build in CI.

• I'm submitting a ...
  [x] bug report
  [ ] feature request
  [ ] question about the decisions made in the repository

• Do you want to request a feature or report a bug?
  bug

• What is the current behavior?
  win build gets stuck

• If the current behavior is a bug, please provide the steps to reproduce and if possible a screenshots and logs of the problem. If you can, show us your code.
  see: https://ci.appveyor.com/project/CherryPy/cheroot/build/291

• What is the expected behavior?
  win build succeeds

• What is the motivation / use case for changing the behavior?
  green CI

• Please tell us about your environment:

• Python version: 3.6/2.7
• Cheroot version: 6.1.1
• CherryPy version: ‒
• OS: Windows
• Browser: ‒

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, e.g. stackoverflow, gitter, etc.)
  build gets stuck at this point

cheroot/server.py::cheroot.server PASSED                                 [  1%]
cheroot/test/test_conn.py::test_HTTP11_persistent_connections 

[webknjaz]

Bugfix v6.1.2 is being released: https://travis-ci.org/cherrypy/cheroot/builds/363823486