eventlet was only referenced within the test_async_eventlet
suite, which would only run if TEST_EVENTLET were set. I've
confirmed that current zuul runs do not set this variable
and these tests have likely been skipped for many years.
As oslo.db has no other eventlet dependency this would make
oslo.db fully compatible with full removal of eventlet from openstack.
Change-Id: Icc33ce47f35b3e6fd813a265b7c337f37b18a57c
f621db86dd