doc: fix sphinx configuration

Fixes QubesOS/qubes-issues#2797

doc/conf.py

@@ -66,7 +66,7 @@ copyright = u'2010-{}, Invisible Things Lab'.format(time.strftime('%Y'))
 # The short X.Y version.
 version = open('../version').read().strip()
 # The full version, including alpha/beta/rc tags.
-release = subprocess.check_output(['git', 'describe', '--long', '--dirty']).strip()
+release = subprocess.check_output(['git', 'describe', '--long', '--dirty']).strip().decode()
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.