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

--- a/contrib/python3-whitelist	Tue Sep 10 12:32:07 2019 -0400
+++ b/contrib/python3-whitelist	Mon Sep 09 14:26:43 2019 -0400
@@ -296,6 +296,7 @@
 test-hgwebdir-paths.py
 test-hgwebdir.t
 test-hgwebdirsym.t
+test-highlight.t
 test-histedit-arguments.t
 test-histedit-base.t
 test-histedit-bookmark-motion.t