Documentation/sphinx: fix kernel-doc extension on python3

Reconcile differences between python2 and python3 on dealing with
stdout, stderr from Popen. This fixes "name 'unicode' is not defined"
errors on python3. We'll need to try to keep the extension working on
both python-sphinx and python3-sphinx so we don't need two copies.
Reported-and-tested-by: NMarius Vlad <marius.c.vlad@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

Documentation/sphinx/kernel-doc.py

@@ -23,6 +23,9 @@
 #
 # Authors:
 #    Jani Nikula <jani.nikula@intel.com>
+#
+# Please make sure this works on both python2 and python3.
+#
 
 import os
 import subprocess
@@ -75,8 +78,10 @@ class KernelDocDirective(Directive):
             p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=True)
             out, err = p.communicate()
 
-            # assume the kernel sources are utf-8
-            out, err = unicode(out, 'utf-8'), unicode(err, 'utf-8')
+            # python2 needs conversion to unicode.
+            # python3 with universal_newlines=True returns strings.
+            if sys.version_info.major < 3:
+                out, err = unicode(out, 'utf-8'), unicode(err, 'utf-8')
 
             if p.returncode != 0:
                 sys.stderr.write(err)