change the __str__ return value for the computer class to string

as the name suggests __str__ needs to be of type string.

docs/docs.org

@@ -208,7 +208,7 @@ class Computer(Device):
                              on_delete=models.PROTECT)
 
     def __str__(self):
-        return self.hostname
+        return str(self.hostname)
 #+END_SRC
 
 ** ComputerDiskRelation and ComputerRamRelation

inventory/models.py

@@ -148,7 +148,7 @@ class Computer(Device):
                              on_delete=models.PROTECT)
 
     def __str__(self):
-        return self.hostname
+        return str(self.hostname)
 
 
 class ComputerDiskRelation(models.Model):