lookup_nonce in abstract class OAuthDatastore of the Python library shows the wrong return type

```
Either that or the example server's MockOAuthDataStore.lookup_nonce is wrong.  
The abstract class 
says to return OAuthToken while MockOAuthDataStore returns an str.

It doesn't seem like the difference would really break anything, but I figured 
you'd want to know 
about the inconsistency.
```

Original issue reported on code.google.com by `sumee...@gmail.com` on 19 Nov 2009 at 12:15
