testapp.py doesn't work if the OpenID provider does the authentication response with a POST request method. It gives an error message: Unsupported method ("POST"). If you try the following OpenID it will reproduce the problem:
To fix, I added a do_POST() method and factored the code for processing the authentication response from do_GET() into a new id_res() method (consistent with openid.mode: id_res). do_POST() reads parameters from the POST body only (as per the spec).
I checked it with the above OpenID and confirmed that it works now.