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: If9104f4b56cd22fed87c596f990350ea877d83b6
Closes-Bug:#1693670
diff --git a/doc/source/conf.py b/doc/source/conf.py
index f11d96a..b9e22b5 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -155,8 +155,7 @@
 git_cmd = ["git", "log", "--pretty=format:'%ad, commit %h'", "--date=local",
    "-n1"]
 try:
-    html_last_updated_fmt = str(
-        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".')