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ARROW-9469: [Python] Make more objects weakrefable

By default, Cython extension classes (defined with "cdef class") don't have a weakref slot, so add one to all of them.

This adds just one memory word to each object, which IMHO is acceptable.

Closes #7758 from pitrou/ARROW-9469-py-weakrefable-objects

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>