bpo-44116: Work around module unloading refleak to unblock buildbots

[encukou]

The csv and urllib.request modules leak references when unloaded using test.support.import_helper.modules_cleanup().
This should unblock refleak buildbots while the issue is investigated.

https://bugs.python.org/issue44116

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit d4f5910 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[erlend-aasland]

Is this still needed?

[encukou]

I don't think it is. Thank you for fixing it thoroughly!