Add a Singleton metaclass, use it with Display (#48935)

* Add a Singleton class, use it with Display

* update six import

* Move remaining failes to display singleton

* Fix rebase issues

* Singleton improvements

* Add code-smell for 'from __main__ import display'. ci_complete

* s/self/cls/g

* Add docs for no-main-display

* Address linting issues

* Add changelog fragment. ci_complete

* Implement reentrant lock for class instantiation in Singleton

* Add Display singleton porting guide

lib/ansible/cli/pull.py

@@ -18,7 +18,6 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
-########################################################
 import datetime
 import os
 import platform
@@ -34,16 +33,11 @@ from ansible.errors import AnsibleOptionsError
 from ansible.module_utils._text import to_native, to_text
 from ansible.plugins.loader import module_loader
 from ansible.utils.cmd_functions import run_cmd
+from ansible.utils.display import Display
 
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+display = Display()
 
 
-########################################################
-
 class PullCLI(CLI):
     ''' is used to up a remote copy of ansible on each managed node,
         each set to run via cron and update playbook source via a source repository.