Fix a test that is failing on Python 3.8b1
Fix a test that is failing on Python 3.8b1

Fedora is doing a mass rebuild of all Python packages in Fedora with
Python 3.8 beta 1 and discovered the Pika tests are failing. The reason
they're failing[0] appears to be a bug in Python 3.8, but this
particular issue makes sense to fix in Pika as well.

The problem is that self.obj.connection is already a MagicMock object,
so a further patch is pointless (and is what is triggering this bug).
The fix is to just not mock the mock object.

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1717665#c5

Signed-off-by: Jeremy Cline <f648af476d5297108517aa3643c2c45a04900eaa@redhat.com>

pika/pika