py3: fix recaptcha Request parameter type urllib.parse.urlencode will (after making '%' encoding of unicode characters) return a str instead of the obviously correct 7-bit ascii bytes. When we need bytes, we thus have to encode to bytes.

# 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
.