create stub for __main__ (#5354)

* create stub for __main__

* exclude from pyright, like all other __getattr__ incomplete stubs

stdlib/VERSIONS

@@ -1,4 +1,5 @@
 __future__: 2.7
+__main__: 2.7
 _ast: 3.6
 _bisect: 2.7
 _bootlocale: 3.6

stdlib/__main__.pyi

@@ -0,0 +1,3 @@
+from typing import Any
+
+def __getattr__(name: str) -> Any: ...