check for deprecation warnings in unit tests

This patch changes deprecation warnings to errors in the testing.TestCase base
class in order to catch features that are used by nutils but due to be removed
by thrd parties.

nutils/testing.py

@@ -184,6 +184,7 @@ def setUp(self):
         self.enter_context(treelog.set(treelog.LoggingLog('nutils')))
         self.enter_context(_builtin_warnings.catch_warnings())
         _builtin_warnings.simplefilter('error', warnings.NutilsWarning)
+        _builtin_warnings.simplefilter('error', DeprecationWarning)
 
     def assertAllEqual(self, actual, desired):
         actual = numpy.asarray(actual)