lib: introduce string conversion functions for ASCII without further encoding concerns Avoid the trial-and-error and vagueness of the "safe" functions. This should replace some use of safe_unicode and safe_str. It will mostly be a noop in py2 but will be crucial 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
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