Removed redundant weakref references

Also tweaked a couple of minor bits (SPI bus init mask) and unused
_get_address override

gpiozero/pins/native.py

@@ -13,7 +13,6 @@ import mmap
 import errno
 import struct
 import warnings
-import weakref
 from time import sleep
 from threading import Thread, Event, Lock
 from collections import Counter