appveyor: Fix Python path in registry.

This makes PyQt pick up C:\Python34 correctly so we can use the newer AppVeyor
image again.

See https://github.com/appveyor/ci/issues/363.

.appveyor.yml

@@ -3,7 +3,6 @@ version: '{branch}-{build}'
 cache:
   - C:\projects\qutebrowser\.cache
 build: off
-os: Previous Windows Server 2012 R2
 environment:
     PYTHON: 'C:\Python34'
     PYTHONUNBUFFERED: 1

scripts/dev/ci_install.py

@@ -33,6 +33,10 @@ import sys
 import subprocess
 import urllib
 
+try:
+    import _winreg as winreg
+except ImportError:
+    winreg = None
 
 
 def apt_get(args):
@@ -59,6 +63,12 @@ if 'APPVEYOR' in os.environ:
         'http://www.qutebrowser.org/pyqt/PyQt5-5.5-gpl-Py3.4-Qt5.5.0-x32.exe',
          r'C:\install-PyQt5.exe')
 
+    print("Fixing registry...")
+    with winreg.OpenKey(winreg.HKEY_CURRENT_USER,
+                        r'Software\Python\PythonCore\3.4', 0,
+                        winreg.KEY_WRITE) as key:
+        winreg.SetValue(key, 'InstallPath', winreg.REG_SZ, r'C:\Python34')
+
     print("Installing PyQt5...")
     subprocess.check_call([r'C:\install-PyQt5.exe', '/S'])