[issue20490] Show clear error message on circular import

[Anthony Sottile]

Change by Anthony Sottile <asottile at umich.edu>:


----------
nosy: +Anthony Sottile
versions: +Python 3.8, Python 3.9 -Python 3.5

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue20490>
_______________________________________