Fix use of 2.7 only assertion.
Fix use of 2.7 only assertion.

jonparrott/oauth2client,googleapis/google-api-python-client,clancychilds/oauth2client,googleapis/google-api-python-client,googleapis/oauth2client,googleapis/oauth2client,google/oauth2client,clancychilds/oauth2client,google/oauth2client,jonparrott/oauth2client