Refs #23919 -- Stopped inheriting from object to define new style classes.

Tests and docs complement to cecc079168.

docs/howto/custom-model-fields.txt

@@ -36,7 +36,7 @@ You only need to know that 52 cards are dealt out equally to four players, who
 are traditionally called *north*, *east*, *south* and *west*.  Our class looks
 something like this::
 
-    class Hand(object):
+    class Hand:
         """A hand of cards (bridge style)"""
 
         def __init__(self, north, east, south, west):

docs/howto/outputting-csv.txt

@@ -71,7 +71,7 @@ the assembly and transmission of a large CSV file::
 
     from django.http import StreamingHttpResponse
 
-    class Echo(object):
+    class Echo:
         """An object that implements just the write method of the file-like
         interface.
         """

docs/howto/writing-migrations.txt

@@ -42,7 +42,7 @@ method of database routers as ``**hints``:
 .. snippet::
     :filename: myapp/dbrouters.py
 
-    class MyRouter(object):
+    class MyRouter:
 
         def allow_migrate(self, db, app_label, model_name=None, **hints):
             if 'target_db' in hints: