py3-discovery: using plain str in stats dict rust-cpython converts automatically from Rust strings to the appropriate `str` for the target Python version. Insisting on discovery stats dict keys to be bytes hence breaks the process (this is spotted by test-setdiscovery.t). Now that byteify-strings has been run on the entire codebase, and the import transformer is not there any more, the simplest fix is to make the keys plain str again. Another possible fix would be to forcefully convert to bytes in rust-cpython code, but that feels less natural, and would probably have to be reverted down the road. Differential Revision: https://phab.mercurial-scm.org/D7039