Add an undocumented "trait" class decorator to mypy_extensions (#5278)

This is an undocumented and extremely experimental annotation to
support a restricted for of multiple inheritance for `mypyc`.

mypy_extensions.py

diff --git a/mypy_extensions.py b/mypy_extensions.py
index 056b1bf..0fc3b43 100644
--- a/mypy_extensions.py
+++ b/mypy_extensions.py
@@ -135,3 +135,7 @@
 
 # Return type that indicates a function does not return
 class NoReturn: pass
+
+
+def trait(cls):
+    return cls