lib: drop own asbool implementation and consistently use tg.support.converters as utils2.asbool str2bool never reported error on odd input such as '' or '-1', but the tg asbool behaviour of raising ValueError("String is not true/false: %r" % obj) in that case seems fine.

# 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
.