media-libs/gegl: Fix compilation error due to non-ascii symbols

media-libs/gegl: Fix compilation error due to non-ascii symbols

This patch force using utf8 while reading files on converting .cl files
to .h files in env where for some reasons sandbox system codepage isn't utf8.

The patch provides compatibility both with python2.7 and python3.x.
When python2.7 support will be dropped the addition of
from __future__ import unicode_literals
import io
and "io."
could be removed as not neccessary for python3.x.

Closes: https://bugs.gentoo.org/705286
Signed-off-by: Sergey Torokhov <torokhov-s-a@yandex.ru>
Closes: https://github.com/gentoo/gentoo/pull/14387
Signed-off-by: Joonas Niilola <juippis@gentoo.org>