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.