[mypyc] Only allow deleting attributes declared using __deletable__

[JukkaL]

Native classes now must declare attributes that can be deleted.

This improves type safety and will make it possible to support
always defined attributes in the future.

Example:

class C:
    x: int
    y: int
    z: int
    __deletable__ = ['x', 'y']

Now x and y can be deleted, but z can't be:

def f(c: C) -> None:
    del c.x  # Ok
    del c.y  # Ok
    del c.z  # Error

__deletable__ has no effect in non-native classes and their behavior is same
as before.

Closes mypyc/mypyc#853.