* Revert "Reverted "Fixed #33213 -- Doc'd testing code coverage in parallel and used it.""
This reverts commit 78da5ca0c1f2ab3201f8f6cd629e80d805ea023d.
* Restored coverage multiprocess concurrency with threads
Investigating https://github.com/nedbat/coveragepy/issues/1585 revealed
that thread tracing gets disabled when passing
`concurrency = multiprocessing`. Adding `thread` restores it, and
ensures that the `auser()` is reported as covered since the test suite
uses `AsyncToSync` to execute this middleware (which spawns threads).
070cbac0db