Documenation/conf.py: Make sure release is a string

With python3 the split method can operate on strings while check_output
generates bytestrings.

Change-Id: I7b455c56e8195f0ecfbe5e360ac161c176f00115
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/27630
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

Documentation/conf.py

@@ -20,7 +20,7 @@ author = u'the coreboot project'
 # built documents.
 #
 # The full version, including alpha/beta/rc tags.
-release = subprocess.check_output(('git', 'describe'))
+release = subprocess.check_output(('git', 'describe')).decode("utf-8")
 # The short X.Y version.
 version = release.split("-")[0]