Context

In the synchronous tests, we should ignore anything that is in an `if not _IS_SYNC` block.
In the asynchronous tests, we should ignore anything that in an `if _IS_SYNC` block.
There isn't a way to do this directly with exclude_lines, so it may be that we have to use synchro.py to handle # pragma: no cover comments to achieve the desired result.

Definition of done

Ignore sync-only code in async files where possible, and vice versa.

Pitfalls

Make sure we're not adding the no cover commands to the wrong places.