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/reboot.py

@@ -13,13 +13,9 @@ from datetime import datetime, timedelta
 from ansible.errors import AnsibleError, AnsibleConnectionFailure
 from ansible.module_utils._text import to_native, to_text
 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()
 
 
 class TimedOutException(Exception):