Allow various UTF formats in the config file if on Python 3.6 or later.

[terrycojones]

The various UTF formats can be handled by the json module if the input file is opened in binary mode. See the note See note at the bottom of https://docs.python.org/3/library/json.html#json.load

The diff here passes rb as a mode if on Python >= 3.6

I don't see anywhere to add tests for this. But... the change works for me and I see the non-ASCII chars correctly displayed by auspice.

Fixes #498.

[elebow]

As of two days ago, Python 3.6 is the minimum supported version (see #482), so the conditional can be omitted.

[terrycojones]

As of two days ago, Python 3.6 is the minimum supported version (see #482), so the conditional can be omitted.

OK, updated.