doc: Fix output of version string when using Sphinx with Python 3

Our Sphinx configuration gets the current skiboot version by using the
subprocess module to run make_version.sh. In Python 2, this returns a
value of type str, but in Python 3, this returns bytes instead.

Decode those bytes into a string so we see "skiboot 5.6.blah" rather
than "skiboot b'5.6.blah\n'" in the documentation output.

Tested using Sphinx with both Python 2 and 3.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

doc/conf.py

@@ -58,9 +58,9 @@ def setup(app):
 #
 # The short X.Y version.
 import subprocess
-version = subprocess.check_output("../make_version.sh | sed -e 's/skiboot-//'", shell=True)
+version = subprocess.check_output("../make_version.sh | sed -e 's/skiboot-//'", shell=True).decode()
 # The full version, including alpha/beta/rc tags.
-release = subprocess.check_output("../make_version.sh | sed -e 's/skiboot-//'", shell=True)
+release = subprocess.check_output("../make_version.sh | sed -e 's/skiboot-//'", shell=True).decode()
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.