make-man-index.py: compatibility with Python 3

Despite its name, ElementTree.tostring() returns already-encoded
bytes() instead of a string in Python 3; sys.stdout doesn't like that.

make-man-index.py

@@ -72,5 +72,7 @@ hr = SubElement(body, 'hr')
 p = SubElement(body, 'p')
 p.text = "This index contains %s entries, referring to %i individual manual pages." % (len(index), len(argv)-1)
 
+if hasattr(stdout, "buffer"):
+	stdout = stdout.buffer
 prettify(html)
 stdout.write(tostring(html))