setup: use modern bcrypt implementation instead of unsupported old one py-bcrypt has been deprecated by bcrypt, and is no longer developed or supported. bcrypt requires bytestrings instead of strings, use safe_str to ensure they're encoded before they're passed to bcrypt. Also, use check_pw to minimise the number of manual conversions and comparisons. Installation of bcrypt will probably compile a C extension and require libffi-dev.

# 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
.