Fix the broken python header __getattr__() behavior, take 13 (or so)

Fix the broken python header  __getattr__() behavior, take 13 (or so)

- Tags as header attributes seemed like a nice idea at the time... but
  has been a PITA due to side-effects it causes, such as breaking
  getattr() use for "capability testing", eg:
      >>> h2 = copy.deepcopy(h)
      Traceback (most recent call last):
        File "<stdin>", line 1, in <module>
        File "/usr/lib64/python2.7/copy.py", line 172, in deepcopy
          copier = getattr(x, "__deepcopy__", None)
      ValueError: unknown header tag
- Since we can't really go removing the brainded feature (somebody might
  actually be using it) try harder to fix it: if its not an actual
  attribute, save the exception we got from PyObject_GenericGetAttr()
  and if its not a valid tag either, restore the original exception.
  This allows cases like the above __deepcopy__ to work properly.