add minimal oauthlib requirement

I get ImportError at /accounts/register/ and /account/login/

```
cannot import name is_secure_transport
../site-packages/requests_oauthlib/oauth2_auth.py in <module>, line 3
```
which seems to be related to https://github.com/requests/requests-oauthlib/issues/139
Upgrading `oauthlib` does help.