Backport PR #6410: Fix relative import in appnope

Closes gh-6409

Ping @minrk - this fix should go into appnope as well.

IPython/external/appnope/__init__.py

@@ -1,15 +1,15 b''
 
 try:
     from appnope import *
 except ImportError:
     __version__ = '0.0.5'
     import sys
     import platform
     from distutils.version import LooseVersion as V
 
     if sys.platform != "darwin" or V(platform.mac_ver()[0]) < V("10.9"):
-        from _dummy import *
+        from ._dummy import *
     else:
         from ._nope import *
 
     del sys, platform, V