python: Rename USB.py -> usb.py (PEP8)

scripts/greaseweazle/USB.py → scripts/greaseweazle/usb.py

@@ -1,4 +1,4 @@
-# greaseweazle/USB.py
+# greaseweazle/usb.py
 #
 # Written & released by Keir Fraser <keir.xen@gmail.com>
 #

scripts/gw.py

@@ -12,7 +12,7 @@ import sys, struct, argparse, serial
 import crcmod.predefined
 
 from greaseweazle import version
-from greaseweazle import USB
+from greaseweazle import usb as USB
 from greaseweazle.bitcell import Bitcell
 from greaseweazle.flux import Flux
 from greaseweazle.scp import SCP