Add support for stateless

[TheZoc]

Hi there,

I did a quick search and found issue #50, where there was a misunderstanding due to a cross reference GitHub automatically made.
Would you consider merging the changes from https://github.com/canonical-web-and-design/flask-openid here, so stateless operation is supported?

Thank you in advance

[vnznznz]

That fork is not in a good shape, for snapcraft.io they went back to the orignal flask-openid with this constructor instead

oid = OpenID(
    store_factory=lambda: None
) 

canonical/snapcraft.io@c341d54