Add lib2to3.__main__ to make it easier for debugging purposes to run 2to3.

Lib/lib2to3/__main__.py

+import sys
+from .main import main
+
+sys.exit(main("lib2to3.fixes"))

Tools/scripts/2to3

 #!/usr/bin/env python
-import sys
-from lib2to3.main import main
+import runpy
 
-sys.exit(main("lib2to3.fixes"))
+runpy.run_module('lib2to3', run_name='__main__', alter_sys=True)