Make list(ui.config) raise a TypeError

As per PEP234 (https://www.python.org/dev/peps/pep-0234/) if an object
has __getitem__() but not __iter__() then the former will be used to
build an iterable, invoking it with 0, then 1, ... until a KeyError is
raised. Lazy configuration makes it never happen, and list(config) runs
indefinitely, hogging all memory.

Implement __iter__(), which raises a TypeError, to solve that problem.

beets/util/confit.py

@@ -210,6 +210,11 @@ class ConfigView(object):
     def __repr__(self):
         return '<ConfigView: %s>' % self.name
 
+    def __iter__(self):
+        """Prevent list(config) from using __getitem__ and never halting"""
+        raise TypeError(u"{!r} object is not "
+                        u"iterable".format(self.__class__.__name__))
+
     def __getitem__(self, key):
         """Get a subview of this view."""
         return Subview(self, key)