Replace `time.sleep()` in tests with a more robust retrying logic.

[facutuesca]

Using this retrying logic might save us some test flakiness in the future

[woodruffw]

My only hesitation here is in upstreamability -- our tests currently avoid pytest because we want to make this straightforward to upstream into CPython itself. Maybe we can carve out the retrying helper here and include it in _utils.py?

[facutuesca]

My only hesitation here is in upstreamability -- our tests currently avoid pytest because we want to make this straightforward to upstream into CPython itself. Maybe we can carve out the retrying helper here and include it in _utils.py?

@woodruffw done! we now have our own retrying helper