unittest: Make AssertRaisesContext store exception for later retrieval.

The statement "with assertRaises(errtype) as ctxt" checks the type of a
raised exception, but did not store the exception into ctxt like unittest
of CPython.  The exception instance is usually used to check its message or
other args.

python-stdlib/unittest/unittest.py

@@ -24,6 +24,8 @@ class AssertRaisesContext:
         if exc_type is None:
             assert False, "%r not raised" % self.expected
         if issubclass(exc_type, self.expected):
+            # store exception for later retrieval
+            self.exception = exc_value
             return True
         return False