adding experimental __slots__ to some classes

[auvipy]

will add more slots and fix the issues which break the CI

related celery/vine#67

[auvipy]

good to see tests are passing

[auvipy]

I want to include it on 5.1 release, may be after christmas or on the new years

[auvipy]

load time without this patch/patches

[auvipy]

@4383 as far i can recall you use this package on old openstack versions right? would love to have you try this changes

[matusvalo]

I don't see any benefit having __slots__ for classes except Message class. __slots__ is mainly performance related and the only class that is created on bigger scales is Message class.

[auvipy]

IMHO, other classes when used in production, could be also benefited as well?

[auvipy]

I am merging this as I can see most of the classes are used by kombu and celery, so in bigger system tiny resource saving would be beneficial, IMHO