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/plugins/action/pause.py

@@ -30,12 +30,9 @@ from ansible.module_utils._text import to_text, to_native
 from ansible.module_utils.parsing.convert_bool import boolean
 from ansible.module_utils.six import PY3
 from ansible.plugins.action import ActionBase
+from ansible.utils.display import Display
 
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+display = Display()
 
 try:
     import curses