Fix SkipRepeatsQueue._put() raises AttributeError (#817)

This fixes an issue with SkipRepeatsQueue._put() which causes it to raise
an AttributeError on the first put. This is done by adding a check to test
if _last_item is set to None, which is the value set by __init__()

src/watchdog/utils/bricks.py

@@ -87,7 +87,7 @@ def _init(self, maxsize):
         self._last_item = None
 
     def _put(self, item):
-        if item != self._last_item:
+        if self._last_item is None or item != self._last_item:
             super()._put(item)
             self._last_item = item
         else: