Do not cast objects when initializing their string representation

closes #3898

(cherry picked from commit 31b2b86)

Author: lubosmj

CHANGES/3898.bugfix

@@ -0,0 +1 @@
+Fixed the method for displaying the representation of ``MasterModel`` objects.

pulpcore/app/models/base.py

@@ -197,18 +197,7 @@ def master(self):
 
     def __str__(self):
         # similar to Model's __str__, but type-aware
-        cast = self.cast()
-        if cast is self:
-            return super().__str__()
-
-        try:
-            return "<{} (pulp_type={}): {}>".format(
-                self._meta.object_name, cast.pulp_type, cast.name
-            )
-        except AttributeError:
-            return "<{} (pulp_type={}): pk={}>".format(
-                self._meta.object_name, cast.pulp_type, cast.pk
-            )
+        return "<{} (pulp_type={}): pk={}>".format(self._meta.object_name, self.pulp_type, self.pk)
 
 
 # Add properties to model _meta info to support master/detail models