model: Make all classes slotted.
Unfortunately, sphinx (actually, autodoc) only picks up attributes if
they fall in any of these cases:
- are enum variants
- are in slots
- are in dict
- have an annotation
- are found using its custom parser
(see get_object_members in sphinx/ext/autodoc/importer.py)
In theory, option 5 should work for us; unfortunately, autodoc only
asks the parser the list of members with a comment.
And it's not easy to adapt it to ask the parser for all members,
because said parser (sphinx/pycode/parser.py) does not return the class
qualname (aka. namespace) for members without comments.
So, as I don't want to change the interface of sphinx.pycode.parser,
this commit switches to relying on option 3, by adding slots for
all attr classes.
Additionally, this might have some performance/memory improvement
(though I did not check) and will further avoid mutation of these
objects.