Reduce closed-event logging; code clean-up

[webbnh]

The current code produces a message like

Attempting to update downstream issue on upstream closed event

on basically every downstream update. This is noisy and confusing (since most updates are not a "closed event").

This PR modifies the code to produce that message only if the issue status actually is "Closed".

And, while we're at it, this PR includes a few other clean-ups:

• Correct type hint references to jira.client which should be jira.client.JIRA
• Remove the unused downstream parameter from match_user()
• After hoisting the is-closed test into the caller, tighten up the rest of the code in _update_on_close(), and enhance the log message there.

And, as it happens, this change causes one of the unit tests to fail, so this PR adds a new unit test which correctly represents the original scenario, and this change updates the original test to test the "issue is now closed" scenario. Also, the original test was creating a mock for _update_assignee() which it wasn't referencing, so I added an assertion for it, as well.

[ralphbean]

Solid, @webbnh. Thanks!