- Some tests that check that assignments are not allowed expect this

to raise TypeError.  In practice, a disallowed attribute assignment
  can raise either TypeError or AttributeError (and it's unclear which
  is better).  So allow either.  (Yes, this is in anticipation of a
  code change that switches the exception raised. :-)

- Add a utility function, cantset(), which verifies that setting a
  particular attribute to a given value is disallowed, and also that
  deleting that same attribute is disallowed.  Use this in the
  test_func_*() tests.

- Add a new set of tests that test conformance of various instance
  method attributes.  (Also in anticipation of code that changes their
  implementation.)