Add `dev-requirements.txt` for pip

`dev-requirements.txt` contains which specific version of Black and
Flake8 to use for development and by a CI so that both parties agree.

Use specific versions as such:

    $ virtualenv -p python3 yo
    $ . yo/bin/activate
    $ pip install -r dev-requirements.txt

Change-Id: Id29e7f61c4038b55bed613e00f76f7c180699fa9
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>