unittest: Allow to catch AssertionError with assertRaises().

Without this change, current implementaiton produces a false positive
result for AssertionError type.

Example of falsely passing test code:

def test(a, b):
    assert a > 10
    assert b > 10

self.assertRaises(AssertionError, test, 20, 20)

python-stdlib/unittest/unittest.py

@@ -126,12 +126,13 @@ class TestCase:
 
         try:
             func(*args, **kwargs)
-            assert False, "%r not raised" % exc
         except Exception as e:
             if isinstance(e, exc):
                 return
             raise
 
+        assert False, "%r not raised" % exc
+
 
 def skip(msg):
     def _decor(fun):