Windows build requires eventlet for some reason (investigate later)

appveyor.yml

@@ -37,6 +37,7 @@ init:
 install:
   - "powershell extra\\appveyor\\install.ps1"
   - "%PYTHON%/Scripts/pip.exe install -U setuptools"
+  - "%PYTHON%/Scripts/pip.exe install -U eventlet"
 
 build: off