Bump the minimum version of py-amqp to 5.1.1

[Herve Beraud]

Few days ago the version 5.1.1 of py-amqp have been released.

Py-amqp 5.0.6 is pulled by python 3.8 and lower versions.

Py-amqp 5.1.1 is required for python 3.9 as the dependency
resolver fail to install lower versions due to a requirement
conflict [2].

This new version of Py-amqp (5.1.1) change the way the
connections are handled, so since this version some of our
tests cases started to failed due to a number of call
that is not the same as with previous versions of py-amqp
due to the way of how the connections are managed.

As lower versions of py-amqp are not compatible with python3.9
these changes propose to use the version 5.1.1 as the minimal
version for all the python versions (3.6, 3.8, 3.9).

Also these changes adapt the tests to match the number of call expected.

[1] https://github.com/celery/py-amqp/compare/v5.1.0...v5.1.1
[2] ERROR: Cannot install amqp==5.1.0 because these package versions
have conflicting dependencies.

The conflict is caused by:
   The user requested amqp==5.1.0
   The user requested (constraint) amqp===5.1.1

To fix this you could try to:
   1. loosen the range of package versions you've specified
   2. remove package versions to allow pip attempt to solve the dependency conflict

[3] oslo.messaging/oslo_messaging/tests/drivers/test_impl_rabbit.py", line 1018, in test_ensure_one_retry
self.assertEqual(3, mock_callback.call_count)

oslo.messaging/.tox/py39/lib/python3.9/site-packages/testtools/testcase.py", line 393, in assertEqual
self.assertThat(observed, matcher, message)

oslo.messaging/.tox/py39/lib/python3.9/site-packages/testtools/testcase.py", line 480, in assertThat
raise mismatch_error

testtools.matchers._impl.MismatchError: 3 != 2

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/oslo.messaging/+/838914

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Hervé Beraud <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/oslo.messaging/+/838914
Reason: Already addressed by https://review.opendev.org/c/openstack/oslo.messaging/+/836678