Fix html_last_updated_fmt for Python3.

html_last_updated_fmt option is interpreted as a
byte string in python3, causing Sphinx build to break.
This patch makes it utf-8 string.

Change-Id: Ifde17e94d0345a289eea29ba3d664d31f1eb51d9
Closes-Bug:#1693670

doc/source/conf.py

@@ -58,8 +58,7 @@ htmlhelp_basename = '%sdoc' % project
 git_cmd = ["git", "log", "--pretty=format:'%ad, commit %h'", "--date=local",
            "-n1"]
 try:
-    html_last_updated_fmt = subprocess.Popen(
-        git_cmd, stdout=subprocess.PIPE).communicate()[0]
+    html_last_updated_fmt = subprocess.check_output(git_cmd).decode('utf-8')
 except Exception:
     warnings.warn('Cannot get last updated time from git repository. '
                   'Not setting "html_last_updated_fmt".')