Prevent querying for double-underscored properties accessed by Python internals.

Python allows classes to define a variety of double-underscore-prefixed names (colloquially referred to as 'dunder' methods) to change the behavior of a class.  One such of these methods, named `__length_hint__`, is accessed in Python 2.6 when performing a select.

This patch fixes this glitch for Python 2.6, and prevents other magic method lookups from triggering similar queries.  Please note that the fact that this will not prevent the `AttributeError` from being raised is intentional -- the raising of that exception is how Python internals are able to tell whether that method is defined.

Cheers, and let me know if you have any questions!