[opt-viewer] Decode HTML bytes for Python 3

[modocache]

Summary

When using Python 3, pygments.highlight() returns a bytes object, not
a str, causing the call to str.replace on the following line to fail
with a runtime exception:
TypeError: 'str' does not support the buffer interface. Decode the
bytes into a string in order to fix the exception.

Test Plan:
Run opt-viewer.py with Python 3.4, and confirm no runtime error occurs
when calling str.replace.

[anemet]

LGTM, with the same note here: as long as it still works with Python2 (which I am less sure of in this case).

[modocache]

I double-checked this diff worked when run on a directory containing a single YAML file, using Python 2, and it appeared to work. Let me know if it breaks anything for any of your workflows!

[lebedev.ri]

This change introduced a regression for python2, which is being fixed by D38289
It would be really nice to get some review there..

tools/opt-viewer/opt-viewer.py

 </html>
             '''.format(filename), file=self.stream)
 
         self.html_formatter = HtmlFormatter(encoding='utf-8')
         self.cpp_lexer = CppLexer(stripnl=False)
 
     def render_source_lines(self, stream, line_remarks):
         file_text = stream.read()
-        html_highlighted = highlight(file_text, self.cpp_lexer, self.html_formatter)
+        html_highlighted = highlight(
+            file_text,
+            self.cpp_lexer,
+            self.html_formatter).decode('utf-8')
 
         # Take off the header and footer, these must be
         #   reapplied line-wise, within the page structure
         html_highlighted = html_highlighted.replace('<div class="highlight"><pre>', '')
         html_highlighted = html_highlighted.replace('</pre></div>', '')
 
         for (linenum, html_line) in enumerate(html_highlighted.split('\n'), start=1):
             print('''