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

test/sanity/code-smell/no-main-display.json

@@ -0,0 +1,10 @@
+{
+    "extensions": [
+        ".py"
+    ],
+    "prefixes": [
+        "bin/",
+        "lib/ansible/"
+    ],
+    "output": "path-line-column-message"
+}

test/sanity/code-smell/no-main-display.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+from __future__ import print_function
+
+import os
+import re
+import sys
+
+MAIN_DISPLAY_IMPORT = 'from __main__ import display'
+
+
+def main():
+    skip = set()
+
+    for path in sys.argv[1:] or sys.stdin.read().splitlines():
+        if path in skip:
+            continue
+
+        with open(path, 'r') as f:
+            for i, line in enumerate(f.readlines()):
+                if MAIN_DISPLAY_IMPORT in line:
+                    lineno = i + 1
+                    colno = line.index(MAIN_DISPLAY_IMPORT) + 1
+                    print('%s:%d:%d: Display is a singleton, just import and instantiate' % (path, lineno, colno))
+
+
+if __name__ == '__main__':
+    main()