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.

pytest >= 4.6.6, < 5.4
pytest-sugar >= 0.9.2, < 0.10
pytest-benchmark >= 3.2.2, < 3.3
pytest-localserver >= 0.5.0, < 0.6
mock >= 3.0.0, < 4.1
Sphinx >= 1.8.0, < 2.4
WebTest >= 2.0.6, < 2.1
isort == 4.3.21
pyflakes == 2.1.1