Message 250250 - Python tracker

Typically it's warnings.warn(message, DeprecationWarning, stacklevel=2), but that stack level is wrong for Python 3.3 - 3.4 (fixed in 3.5.0). If this is purely for Python 3.6 I might be adding a nicer API for module deprecations, but if this is for other versions then the call above is the right one except for Python 3.3/3.4.