tests: fix get-with-headers.py on python3 when writing to stdout
Differential Revision: https://phab.mercurial-scm.org/D2306
<?xml version="1.0" encoding="{encoding}"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US" lang="en-US">
<head>
<link rel="icon" href="{staticurl|urlescape}hgicon.png" type="image/png" />
<meta name="robots" content="index, nofollow"/>
<link rel="stylesheet" href="{staticurl|urlescape}style-gitweb.css" type="text/css" />
<script type="text/javascript" src="{staticurl|urlescape}mercurial.js"></script>