Work around for longer object lifetimes on pypy

On pypy the subordinate LED objects in RGBLED composite objects do die
on failed construction ... eventually. Unfortunately it's not quick
enough to prevent the following tests from failing. As we can't know for
certain exactly which test is going to follow, it's best to simply clear
down the reservation table before each test.

tests/test_boards.py

@@ -34,6 +34,7 @@ def setup_function(function):
 
 def teardown_function(function):
     Device._pin_factory.reset()
+    Device._reservations.clear()
 
 def teardown_module(module):
     # make sure we reset the default