Last minute changes for 1.2

Warnings about non-physical pins, and period specification for
sin/cosine waves in tools

gpiozero/__init__.py

@@ -51,6 +51,8 @@ from .exc import (
     GPIOZeroWarning,
     SPIWarning,
     SPISoftwareFallback,
+    PinWarning,
+    PinNonPhysical,
 )
 from .devices import (
     Device,