Make `NullHandler` removable under py2.6.

When calling `logger.removeHandler(h)` with one of our custom
`NullHandler` instances, things crashed.

diceware/logger.py

@@ -43,6 +43,12 @@ except ImportError:  # NOQA  # pragma: no cover
         def createLock(self):
             pass
 
+        def acquire(self):
+            pass
+
+        def release(self):
+            pass
+
 
 #: Logger that can be used for all diceware related messages.
 logger = logging.getLogger("ulif.diceware")

tests/test_logger.py

 import logging
-from diceware.logger import logger, configure
+from diceware.logger import logger, configure, NullHandler
+
+
+def test_null_handler_removable():
+    # the self-made NullHander is removable under py2.6
+    my_logger = logging.getLogger('foo')
+    handler = NullHandler()
+    my_logger.addHandler(handler)
+    assert len(my_logger.handlers) == 1
+    my_logger.removeHandler(handler)
+    assert len(my_logger.handlers) == 0
 
 
 def test_logger_exists():