Add caching to OAuthAPIClient
Adds internal caching of access tokens to OAuthAPIClient using the Django-based TieredCache.
Note: Also drops Python 3.6 testing in favor of Python 3.5 and 3.8.
BREAKING CHANGE:
- The addition of the TieredCache means that Django is required, as well as certain custom middleware. See the README update for details.
- Removed
OAuthAPIClient.oauth_uri
.