Add __slots__ where appropriate to data classes.

discord/mixins.py

@@ -25,6 +25,8 @@ DEALINGS IN THE SOFTWARE.
 """
 
 class EqualityComparable:
+    __slots__ = []
+
     def __eq__(self, other):
         return isinstance(other, self.__class__) and other.id == self.id
 
@@ -34,5 +36,7 @@ class EqualityComparable:
         return True
 
 class Hashable(EqualityComparable):
+    __slots__ = []
+
     def __hash__(self):
         return hash(self.id)