The async_auth_flow override was being driven via 'await' in httpx's
async dispatcher, which yielded 'NoneType is not awaitable' because a
plain generator yielding a Request doesn't produce an awaitable.
httpx.Auth has three hooks: sync_auth_flow, async_auth_flow, and the
generic auth_flow. The default sync/async implementations delegate to
auth_flow when subclasses override only that one, which is exactly the
behaviour we want: one plain-generator implementation shared across
sync and async clients. Override auth_flow, drop sync/async overrides.
Robert Helewka
66b5dd7bdd