lib: establish py3 compatible strategy for string handling: introducing safe_bytes and deprecating safe_str The meaning of safe_str will change when moving to py3. All use of safe_str is thus tech debt that we have to chop off, mostly by moving to either safe_unicode or safe_bytes ... or dropping because we know what we are doing and rely on the improved type safety in py3.

# 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
.