inspect: return defargs[0] if obj.__dict__ raises exception

The fallback implemented in #2731 cannot return `obj.__dict__[name]`
if the `__dict__` method has been redefined in such a way that it
raises an exception when trying to access it.

This commit adds a try-except block to work around this.

tests/test_util_inspect.py

@@ -0,0 +1,58 @@
+# -*- coding: utf-8 -*-
+"""
+    test_util_inspect
+    ~~~~~~~~~~~~~~~
+
+    Tests util.inspect functions.
+
+    :copyright: Copyright 2007-2016 by the Sphinx team, see AUTHORS.
+    :license: BSD, see LICENSE for details.
+"""
+from unittest import TestCase
+
+from sphinx.util import inspect
+
+
+class TestSafeGetAttr(TestCase):
+    def test_safe_getattr_with_default(self):
+        class Foo(object):
+            def __getattr__(self, item):
+                raise Exception
+
+        obj = Foo()
+
+        result = inspect.safe_getattr(obj, 'bar', 'baz')
+
+        assert result == 'baz'
+
+    def test_safe_getattr_with_exception(self):
+        class Foo(object):
+            def __getattr__(self, item):
+                raise Exception
+
+        obj = Foo()
+
+        with self.assertRaisesRegexp(AttributeError, 'bar'):
+            inspect.safe_getattr(obj, 'bar')
+
+    def test_safe_getattr_with_property_exception(self):
+        class Foo(object):
+            @property
+            def bar(self):
+                raise Exception
+
+        obj = Foo()
+
+        with self.assertRaisesRegexp(AttributeError, 'bar'):
+            inspect.safe_getattr(obj, 'bar')
+
+    def test_safe_getattr_with___dict___override(self):
+        class Foo(object):
+            @property
+            def __dict__(self):
+                raise Exception
+
+        obj = Foo()
+
+        with self.assertRaisesRegexp(AttributeError, 'bar'):
+            inspect.safe_getattr(obj, 'bar')