Remove overlapping slots from AbstractKey subclasses

`PublicKey` and `PrivateKey` both define the `n` and `e` slots, which
are already present in their base class. This reduces the benefits of
having slots.

```shell
$ slotscheck -m rsa -v
ERROR: 'rsa.key:PrivateKey' defines overlapping slots.
       - e (rsa.key:AbstractKey)
       - n (rsa.key:AbstractKey)
ERROR: 'rsa.key:PublicKey' defines overlapping slots.
       - e (rsa.key:AbstractKey)
       - n (rsa.key:AbstractKey)
```

The Python docs say:

> If a class defines a slot also defined in a base class, the instance
> variable defined by the base class slot is inaccessible (except by
> retrieving its descriptor directly from the base class). This renders
> the meaning of the program undefined.

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