Create tz-aware datetime objects

So far the `OAuth2Authentication` backend created naive `datetime` objects when
authenticating the provided credentials.

`django-oauth2-provider` handles tz-aware datetimes since version
0.2.5 (cf [commit 343e422ce1][0]).

This commit uses `django.utils.timezone` to generate tz-aware datetimes when
verifying that a presented `AccessToken` is valid (as in not yet expired).

 [0]: https://github.com/caffeinehit/django-oauth2-provider/commit/343e422ce1963d3fdcdeeab786338b35b0e01732