Allow final methods in non-final cdef classes

[robertwb]

In Cython 0.17.2, the following

cdef class Foo:
    cpdef inline bar(self):
        pass

fails to compile with an ''Only final types can have final Python (def/cpdef) methods'' error.

Ticket http://trac.cython.org/ticket/586: ''Support "final" methods in non-final cdef classes'' suggests that this should work, though. Said ticket also implemented an ''Overriding final methods is not allowed'' error which would be the expected result if one were to override a final method.

Robert Bradshaw wrote (https://groups.google.com/d/msg/cython-users/UuQvpvdNhjc/DILSEQvOun4J): The issue is how to prevent them from being overridden in Python (which is not unsolveable, could probably be done pretty easily with a metaclass). It's intentional that we disallow it before such a mechanism is in place

Migrated from http://trac.cython.org/ticket/795