demandimport: add __main__ to the blacklist (because of setuptools)

mercurial/demandimport.py

@@ -130,6 +130,9 b' ignore = ['
     'resource',
     # this trips up many extension authors
     'gtk',
+    # setuptools' pkg_resources.py expects "from __main__ import x" to
+    # raise ImportError if x not defined
+    '__main__',
     ]
 
 def enable():