[appveyor] must install requirements/dev.txt

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 -r requirements/dev.txt"
 
 build: off