test_generator_raises_error unit tests fails with stevedore > 2.0.1

[yatin]

Detected while bumping oslo policy to 3.3.1 in RDO[1], Tests fails with below traceback, Failing in upstream gates too, tested via https://review.opendev.org/#/c/741869/. Tests pass with stevedore <= 2.0.1, need to make it compatible with stevedore> 2.0.1

DEBUG: ==============================
DEBUG: Failed 1 tests - output below:
DEBUG: ==============================
DEBUG: oslo_policy.tests.test_generator.GeneratorRaiseErrorTestCase.test_generator_raises_error
DEBUG: ----------------------------------------------------------------------------------------
DEBUG: Captured traceback:
DEBUG: ~~~~~~~~~~~~~~~~~~~
DEBUG: Traceback (most recent call last):
DEBUG: File "/builddir/build/BUILD/oslo.policy-3.3.1/oslo_policy/tests/test_generator.py", line 498, in test_generator_raises_error
DEBUG: fake_ep.name)
DEBUG: File "/usr/lib/python3.6/site-packages/testtools/testcase.py", line 485, in assertRaises
DEBUG: self.assertThat(our_callable, matcher)
DEBUG: File "/usr/lib/python3.6/site-packages/testtools/testcase.py", line 498, in assertThat
DEBUG: raise mismatch_error
DEBUG: testtools.matchers._impl.MismatchError: <function _generate_sample at 0x7fb311096c80> returned None

https://logserver.rdoproject.org/02/28602/2/check/rdoinfo-DLRN-check-centos8/8704d48/buildset/centos-rpm-master/repos/component/common/ca/b2/cab28649c689067970a51a2f9b329bdd6a0f0501_dev/mock.log

[yatin (yatinkarel)]

Proposed Fix: https://review.opendev.org/#/c/741869/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/741869
Committed: https://git.openstack.org/cgit/openstack/oslo.policy/commit/?id=6296fce9f8e3205506f87dd31d0b18f4b7842000
Submitter: Zuul
Branch: master

commit 6296fce9f8e3205506f87dd31d0b18f4b7842000
Author: yatinkarel <email address hidden>
Date: Tue Jul 21 09:55:42 2020 +0530

    Fix unit tests to work with stevedore > 2.0.1

    stevedore has switched to importlib_metadata[1] and
    this breaked unit test as the test relied on internal
    implementation of it.

    Instead we should switch to mock NamedExtensionManager
    that's what called by oslo_policy.

    [1] https://review.opendev.org/#/c/739306/

    Closes-Bug: #1888208
    Change-Id: I993d743c53fa3506ceda3d1f291c12f4635eb60a

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/742589

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/742589
Committed: https://git.openstack.org/cgit/openstack/oslo.config/commit/?id=a2652b3584da06e4c4ebe96261672309b01c1122
Submitter: Zuul
Branch: master

commit a2652b3584da06e4c4ebe96261672309b01c1122
Author: yatinkarel <email address hidden>
Date: Thu Jul 23 11:05:05 2020 +0530

    Fix unit tests to work with stevedore > 2.0.1

    stevedore has switched to importlib_metadata[1] and
    this breaked unit test as the test relied on internal
    implementation of it.

    Instead we should switch to mock NamedExtensionManager
    that's what called by oslo_config.

    [1] https://review.opendev.org/#/c/739306/

    Closes-Bug: #1888208
    Change-Id: Ic4b990a387d875a351c9f8e7eaaef726734ff305