python: make the DictImporter's unload() work in any context.

import sys since sys may not be defined in whatever context the DictImporter
is used.  Also reset self.installed after an unload since the same DictImporter
could be used again

--HG--
extra : convert_revision : 988ed7ad8cd41b69e8fc583e618b1b4a146216da

src/python/generate.py

@@ -46,8 +46,10 @@ class DictImporter(object):
         self.unload()
 
     def unload(self):
+        import sys
         for module in self.installed:
             del sys.modules[module]
+        self.installed = set()
 
     def find_module(self, fullname, path):
         if fullname == '__scons':