highlight: fix encoding issues to enable Py3 compatibility This commit fixes various encoding issues with the `highlight` extension to enable compatibility with Python 3. Python `.encode()` and `.decode()` requires the target encoding to be passed as a `str`, so the value of `mercurial.encoding.encoding` must be converted before passing to the function. Pygments also assumes the `str` type for values it works with, so we must perform conversions before and after receiving values from its APIs. After applying this patch, `test-highlight.t` passes under Python 3. We add it to `python3-whitelist` as well. Tested with Pygments 2.4.2. Differential Revision: https://phab.mercurial-scm.org/D6832