Make __del__ methods not rely on globals.
Make __del__ methods not rely on globals.

Globals might be garbage collected before instances upon interpreter
shutdown. So bind them to class, itself bound to its instances, to have a
safe place to access them from.
Also, get rid of USBTransferHelper.__del__, as its job will anyway happen
when USBTransfer.__del__ gets called (if a transfer was provided upon
construction, which is deprecated).

vpelletier/python-libusb1,vpelletier/python-libusb1