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.

rsa/key.py

@@ -239,7 +239,7 @@ class PublicKey(AbstractKey):
 
     """
 
-    __slots__ = ("n", "e")
+    __slots__ = ()
 
     def __getitem__(self, key: str) -> int:
         return getattr(self, key)
@@ -404,7 +404,7 @@ class PrivateKey(AbstractKey):
 
     """
 
-    __slots__ = ("n", "e", "d", "p", "q", "exp1", "exp2", "coef")
+    __slots__ = ("d", "p", "q", "exp1", "exp2", "coef")
 
     def __init__(self, n: int, e: int, d: int, p: int, q: int) -> None:
         AbstractKey.__init__(self, n, e)