$.$ Activity Class $($Base Class$)$ Serves as the base class for user activities. Contains common attributes like

the user associated with the activity and the content of the activity. Both Post and Message classes

inherit from Activity.

Variables:

user $($User$)$: The user associated with the activity.

content $($str$)$: Content of the activity.

Methods:

$\_\_$init$\_\_($self$,$ user, content$)$: Initialize an Activity instance.

$\_\_$str$\_\_($self$)$: Return a string representation of the Activity.