Rework when_changed attribute to use weakrefs

Some fairly major changes to ensure that the Pin.when_changed property
doesn't keep references to the objects owning the callbacks that are
assigned. This is vaguely tricky given that ordinary weakref's can't be
used with bound methods (which are ephemeral), so I've back-ported
weakref.WeakMethod from Py3.4.

This solves a whole pile of things like Button instances not
disappearing when they're deleted, and makes composite devices
containing Buttons much easier to construct as we don't need to worry
about partially constructed things not getting deleted.

gpiozero/pins/pigpiod.py

@@ -6,9 +6,15 @@ from __future__ import (
     )
 str = type('')
 
-import weakref
-import pigpio
 import os
+from weakref import proxy
+from threading import RLock
+try:
+    from weakref import WeakMethod
+except ImportError:
+    from .compat import WeakMethod
+
+import pigpio
 
 from . import SPI
 from .pi import PiPin, PiFactory
@@ -164,6 +170,7 @@ class PiGPIOPin(PiPin):
         self._pull = 'up' if factory.pi_info.pulled_up(self.address[-1]) else 'floating'
         self._pwm = False
         self._bounce = None
+        self._when_changed_lock = RLock()
         self._when_changed = None
         self._callback = None
         self._edges = pigpio.EITHER_EDGE
@@ -269,26 +276,24 @@ class PiGPIOPin(PiPin):
         finally:
             self.when_changed = f
 
-    def _get_when_changed(self):
-        if self._callback is None:
-            return None
-        return self._callback.callb.func
+    def _call_when_changed(self, gpio, level, tick):
+        super(PiGPIOPin, self)._call_when_changed()
 
-    def _set_when_changed(self, value):
+    def _enable_event_detect(self):
+        self._callback = self.factory.connection.callback(
+                self.number, self._edges, self._call_when_changed)
+
+    def _disable_event_detect(self):
         if self._callback is not None:
             self._callback.cancel()
             self._callback = None
-        if value is not None:
-            self._callback = self.factory.connection.callback(
-                    self.number, self._edges,
-                    lambda gpio, level, tick: value())
 
 
 class PiGPIOHardwareSPI(SPI, Device):
     def __init__(self, factory, port, device):
         self._port = port
         self._device = device
-        self._factory = weakref.proxy(factory)
+        self._factory = proxy(factory)
         super(PiGPIOHardwareSPI, self).__init__()
         self._reserve_pins(*(
             factory.address + ('GPIO%d' % pin,)
@@ -382,7 +387,7 @@ class PiGPIOSoftwareSPI(SPI, Device):
         self._clock_pin = clock_pin
         self._mosi_pin = mosi_pin
         self._miso_pin = miso_pin
-        self._factory = weakref.proxy(factory)
+        self._factory = proxy(factory)
         super(PiGPIOSoftwareSPI, self).__init__()
         self._reserve_pins(
             factory.pin_address(clock_pin),