PYTHON-5386 Better test assertions for isinstance

[sleepyStick]

Oops accidentally had the wrong ticket number...(only realized after i had created a draft PR >.<)
But this just changes assertions for isinstance calls:
self.assertTrue(isinstance(..., ...)) -> self.assertIsInstance(..., ...)
self.assertFalse(isinstance(..., ...)) -> self.assertNotIsInstance(..., ...)

[evergreen-ci-prod]

There is an existing patch(es) for this commit SHA:

• Evergreen patch with PR

Please note that the status that is posted is not in the context of this PR but rather the (latest) existing patch and that may affect some tests that may depend on the particular PR. If your tests do not rely on any PR-specific values (like base or head branch name) then your tests will report the same status. If you would like a patch to run in the context of this PR and abort the other(s), comment 'evergreen retry'.

[NoahStapp]

Can you make a sub-task on the JIRA ticket for this work? Tracking each assertion type improvement separately would be helpful.

[sleepyStick]

Can you make a sub-task on the JIRA ticket for this work? Tracking each assertion type improvement separately would be helpful.

yes, good call, done! Just renamed the PR name to use the new ticket / subtask number.

[NoahStapp]

Great work! Please update the ticket in the title to the correct PYTHON-5386 before merging.