tools.memories: define __eq__()/__ne__() of SopelIdentifierMemory

At this time, two `SopelIdentifierMemory` objects are considered equal
if their keys and values compare as equal, even if they use different
`make_identifier` functions. This could change in the future, but seems
good enough for now.

sopel/tools/memories.py

@@ -273,3 +273,11 @@ def __ior__(self, other):
             return NotImplemented
         self.update(other)
         return self
+
+    def __eq__(self, other):
+        if not isinstance(other, dict):
+            return NotImplemented
+        return super().__eq__(other)
+
+    def __ne__(self, other):
+        return not self.__eq__(other)