Backport PR #5657: use 'localhost' as default for the notebook server

instead of loopback IP

seems to workaround weird issues with Sophos on Windows, and should behave the same everywhere else.

IPython/html/notebookapp.py

@@ -86,7 +86,6 b' from IPython.kernel.zmq.kernelapp import ('
 )
 from IPython.nbformat.sign import NotebookNotary
 from IPython.utils.importstring import import_item
-from IPython.utils.localinterfaces import localhost
 from IPython.utils import submodule
 from IPython.utils.traitlets import (
     Dict, Unicode, Integer, List, Bool, Bytes,
@@ -343,11 +342,9 b' class NotebookApp(BaseIPythonApplication):'
 
     # Network related information.
 
-    ip = Unicode(config=True,
+    ip = Unicode('localhost', config=True,
         help="The IP address the notebook server will listen on."
     )
-    def _ip_default(self):
-        return localhost()
 
     def _ip_changed(self, name, old, new):
         if new == u'*': self.ip = u''
@@ -658,7 +655,7 b' class NotebookApp(BaseIPythonApplication):'
 
     @property
     def connection_url(self):
-        ip = self.ip if self.ip else localhost()
+        ip = self.ip if self.ip else 'localhost'
         return self._url(ip)
 
     def _url(self, ip):