all builtin types are subclassable, even when they shouldn't be

[toshok]

the following example:

class foo(slice):
    pass

succeeds in pyston and raises in cpython:

TypeError: Error when calling the metaclass bases
    type 'slice' is not an acceptable base type

this is due to the fact that Py_TPFLAGS_BASETYPE is or'ed into the tp_flags of all BoxedClasses. we need a way to either keep that from happening or override it after BoxedClass creation time. we also need to audit our builtins to make sure only the proper ones are subclassable.

[undingen]

while slice is now fixed there are still some types left:
e.g:

class C(type(xrange(1))): pass

[undingen]

fixed in #1061