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/__init__.py

@@ -40,16 +40,13 @@ from ansible.module_utils.six import with_metaclass, string_types
 from ansible.module_utils._text import to_bytes, to_text
 from ansible.parsing.dataloader import DataLoader
 from ansible.release import __version__
+from ansible.utils.display import Display
 from ansible.utils.path import unfrackpath
 from ansible.utils.vars import load_extra_vars, load_options_vars
 from ansible.vars.manager import VariableManager
 from ansible.parsing.vault import PromptVaultSecret, get_file_vault_secret
 
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+display = Display()
 
 
 class SortedOptParser(optparse.OptionParser):