mercurial: support Mercurial 5.5 No release notes yet, but testing shows it working with a few changes. A few tests need updating because error messages from remote ssh like our sys.stderr.write('abort: %s\n' % error) like for remote: abort: Access to %r denied will now appear on stderr instead of stdout - a very reasonable bug fix.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.