[mypyc] Move setting of __cpyfunction__ outside of constructor

[p-sawicki]

When the __cpyfunction__ attribute of callable class objects is initialized in the constructor, it might end up becoming NULL after a freed instance is reused because the attribute gets cleared when deallocating the object and the constructor would only initialize it when creating a new instance.

To fix this, move the initialization outside of the constructor and do it right after. This also makes it consistent with the __mypyc_env__ attribute.