setup: use sysstr() on process output Otherwise we get a str-bytes mismatch on Python 3 if an error occurs. Differential Revision: https://phab.mercurial-scm.org/D8469

automation: install latest Python versions in Linux Staying up to date. Keeping parity with the Windows environment. Differential Revision: https://phab.mercurial-scm.org/D8467

contrib: update to latest Python 2.7, 3.7, and 3.8 We would ideally update the 3.5 and 3.6 versions as well. But Python didn't generate exe installers for newer versions for some reason. Differential Revision: https://phab.mercurial-scm.org/D8466

automation: always use latest Windows AMI The old AMI isn't available any more. We seem to run into this problem every few months. Amazon (or Microsoft) appears to be removing the old AMIs when they are superseded or something. Let's give up on tracking known images and switch the image selection logic to use the latest published image. Differential Revision: https://phab.mercurial-scm.org/D8465

rust-chg: reimplement attach_io operation as async function In short, MessageLoop<Connection> was redesigned as Protocol<Connection>, and the protocol methods no longer consume self. API changes are briefly documented in the following page: https://docs.rs/tokio-hglib/0.3.0/tokio_hglib/struct.Protocol.html Differential Revision: https://phab.mercurial-scm.org/D8442

rust-chg: upgrade to futures-0.3 based libraries And do some trivial fixes: - BytesMut::put_u32_be() -> put_u32() - tokio_process -> tokio::process, CommandExt -> Command, spawn_async() -> spawn(), stdin() -> stdin - tokio_timer::sleep() -> tokio::time::delay_for() Differential Revision: https://phab.mercurial-scm.org/D8441