Fix unicode vcs changelog encode err

[davidlatwe]

Problem

rez-release raise UnicodeDecodeError when releasing package with non-ascii characters in change log.

Solution

• Explicitly set encoding="utf-8" when fetching results from vsc with subprocess. (but not if python<3.6)
• Allow non-ascii character when formatting with json.

[nerdvegas]

I'm not sure this is right. Looking at the original code, I think there may have been an existing bug, and L76 should be:

if sys.version_info[:2] >= (3, 6) and "encoding" not in kwargs:

[davidlatwe]

Ah, my original intention was to not pin the encoding to "utf-8" (no specific reason), only removes it when the Python version is not compat, since encoding kwarg has been explicit set in commit d38e873, which should be removed if Python version incompat.

Now after #961, I think commit d38e873 should be reverted or it would be Python 2.7 incompat ?