oslo.log: test failures with Python 3.13.0b2

[Michał Górny]

When running the test suite on Python 3.13.0b2, I'm getting the following test failures (note that I had to remove the test requiring eventlet, as greenlet doesn't build on py3.13):

```
oslo_log.tests.unit.test_log.ContextFormatterTestCase.test_rfc5424_isotime_format_no_microseconds
-------------------------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/usr/lib/python3.13/unittest/mock.py", line 1420, in patched
    return func(*newargs, **newkeywargs)

      File "/tmp/oslo.log/oslo_log/tests/unit/test_log.py", line 1003, in test_rfc5424_isotime_format_no_microseconds
    self.assertEqual(expected, self.stream.getvalue())
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/tmp/oslo.log/.tox/py313/lib/python3.13/site-packages/testtools/testcase.py", line 419, in assertEqual
    self.assertThat(observed, matcher, message)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/tmp/oslo.log/.tox/py313/lib/python3.13/site-packages/testtools/testcase.py", line 509, in assertThat
    raise mismatch_error

    testtools.matchers._impl.MismatchError: !=:
reference = '''\
2015-12-16T13:54:26.000000+00:00 test
'''
actual = '''\
2015-12-16T13:54:26+00:00 test
'''

oslo_log.tests.unit.test_log.MutateTestCase.test_remove_logger
--------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/tmp/oslo.log/oslo_log/tests/unit/test_log.py", line 1593, in test_remove_logger
    log.warn("warn")
    ^^^^^^^^

    AttributeError: 'Logger' object has no attribute 'warn'
```

I'm going to try to submit a fix for these.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/oslo.log/+/921797

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/oslo.log/+/921798

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Michał Górny <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/oslo.log/+/921798
Reason: Incorrect solution.