Google has several identity solutions, see my answer to this Q&A, check which is best suited for your app:
What is the difference between Google identity toolkit, Google OAauth and Google+ sign in
In particular the Identity Toolkit offers federated login (including Yahoo) and does offer access to the user email. A bit tricky to use on GAE, tho as the module and its dependencies need to be vendored in. This is how my lib dir looks like with all its dependencies I found:
> ls -la lib/
total 12
drwxrwxr-x 2 userid at 4096 Jul 8 12:42 .
drwxrwxr-x 8 userid at 4096 Jul 17 00:54 ..
lrwxrwxrwx 1 userid at 49 Jul 8 12:42 httplib2 -> /usr/local/google_appengine/lib/httplib2/httplib2
lrwxrwxrwx 1 userid at 85 Jul 8 12:40 identitytoolkit -> /usr/local/google_appengine_lib/identity-toolkit-python-client-master/identitytoolkit
lrwxrwxrwx 1 userid at 57 Jul 8 12:41 oauth2client -> /usr/local/google_appengine/lib/oauth2client/oauth2client
lrwxrwxrwx 1 userid at 53 Jul 8 12:42 simplejson -> /usr/local/google_appengine/lib/simplejson/simplejson
lrwxrwxrwx 1 userid at 39 Jul 8 12:42 six -> /usr/local/google_appengine/lib/six/six
Note: instead of pip-installing the dependencies (I dislike that recipe) I just symlinked in the respective modules from the GAE SDK itself.