[#636769] Fix for major rexec bugs

* Lib/rexec.py
  (FileBase): Added 'xreadlines' and '__iter__' to allowed file methods.
  (FileWrapper.__init__): Removed unnecessary self.f variable, which gave
  direct access to the file object.
  (RExec): Added 'xreadlines' and '_weakref' to allowed modules.
  (RExec.r_open): Convert string subclasses to a real string classes
  before doing comparisons with mode parameter.

* Lib/ihooks.py
  (BasicModuleImporter.import_module/reload/unload): Convert the module
  name to a real string before working with it.
  (ModuleImporter.import_module/import_it/reload): Convert the module
  name to a real strings before working with it.

* Misc/NEWS
  Document the change.