Use Self annotation in store self-partialing

[rsokl]

A subclassed ZenStore will now return the correct type -- the subclass -- when self-partialing. This works for both pyright and mypy.

E.g.

from hydra_zen.wrapper import ZenStore

class MyStore(ZenStore):
    ...

store = MyStore()
optim_store = store(group="optim")

# Before: type checker sees `ZenStore`
# After: type checker sees `MyStore`
reveal_type(optim_store)