Move __init__ from BaseSet into Set and ImmutableSet.  This causes a tiny amount of code duplication, but makes it possible to give BaseSet an __init__ that raises an exception.
Move __init__ from BaseSet into Set and ImmutableSet.  This causes a
tiny amount of code duplication, but makes it possible to give BaseSet
an __init__ that raises an exception.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator