lib: introduce string conversion functions for ASCII without further encoding concerns Avoid the trial-and-error and vagueness of the "safe" functions. This should replace some use of safe_unicode and safe_str. It will mostly be a noop in py2 but will be crucial in py3.

pytest >= 4.6.6, < 4.7
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, < 3.1
Sphinx >= 1.8.0, < 1.9
WebTest >= 2.0.3, < 2.1
isort == 4.3.21