[Code] Python library only supports assertions sent as GET (not POST)

[Jack Bates]

According to OpenID specification, indirect messages can be sent as a GET or as
a POST. However the Python library doesn't support assertions which are sent as
a POST - it only supports assertions which are sent as a GET. Consequently the
Python library fails to interoperate with some OpenID providers which
nonetheless conform to the OpenID specification

I opened this pull request to address this issue:
https://github.com/openid/python-openid/pull/25