lib: only maintain one copy of safe_str / safe_unicode The standalone-ish nature of vcs gets a bit in the way. It already depends on some very generic Kallithea functionality. But for now, avoid code duplication, and let Kallithea use vcs functionality instead of duplicating it.

# 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
.