Investigate/incorporate/collaborate on Debian packaging & testing

[moreati]

Debian packages Mitogen as python3-mitogen and ansible-mitogen. They carry a number of patches to enable this.

• What upstream changes here would make Debian's job easier?
• What automated testing does Debian do?
• What bugs has Debian uncovered? ETA: https://bugs.debian.org/cgi-bin/pkgreport.cgi?package=ansible-mitogen

refs #833

[moreati]

refs #1118

[moreati]

https://sources.debian.org/patches/python-mitogen/0.3.23-2/skip-python2.7-test/ can probably be dropped, it should be covered by

@unittest.skipIf(
    not testlib.have_python2() or not testlib.have_python3(),
    "Python 2/3 compatibility tests require both versions on the controller",
)

added in https://github.com/mitogen-hq/mitogen/commit/924dbd6f0c31f1169adc8d969115dbcdfecb3cc9