Need for speed: Patch #921466 : sys.path_importer_cache is now used to cache valid and

Need for speed: Patch #921466 : sys.path_importer_cache is now used to cache valid and
  invalid file paths for the built-in import machinery which leads to
  fewer open calls on startup.

  Also fix issue with PEP 302 style import hooks which lead to more open()
  calls than necessary.