The html_last_updated_fmt option is the output of a shell command, which
is interpeted as a byte string in python 3 and subsequently breaks
Sphinx.
This change converts it to a utf-8 string.
Incidentally, this also removes the quotation marks to make the
formatted output cleaner.
Change-Id: I9aa6446a24d2f340246458dea0c34d456b3eade3
Closes-bug: #1583721
a496a6bae0