[LNT] Python 3 support: use chr instead of unichr

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Summary

Use Python3's chr instead of Python2's unichr by importing it from
builtins. This was produced by running futurize's stage2
libfuturize.fixes.fix_unicode_keep_u.

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Patch no longer necessary after rebasing a later patch that rewrite this code

tests/server/ui/V4Pages.py

 #
 # RUN: lnt import %t.instance %S/Inputs/sample-failed-report1.json
 # RUN: lnt import %t.instance %S/Inputs/sample-failed-report2.json
 #
 # RUN: python %s %t.instance %{tidylib}
 
 from future import standard_library
 standard_library.install_aliases()
+from builtins import chr
 import logging
 import re
 import sys
 import xml.etree.ElementTree as ET
 from html.entities import name2codepoint
 from flask import session
 import lnt.server.db.migrate
 import lnt.server.ui.app
def dump_html(html_string):
     for linenr, line in enumerate(html_string.split('\n')):
         print("%4d:%s" % (linenr + 1, line))
 
 
 def get_xml_tree(html_string):
     try:
         parser = ET.XMLParser()
         parser.parser.UseForeignDTD(True)
-        parser.entity.update((x, unichr(i)) for x, i in name2codepoint.items())
+        parser.entity.update((x, chr(i)) for x, i in name2codepoint.items())
         tree = ET.fromstring(html_string, parser=parser)
     except:  # noqa FIXME: figure out what we expect this to throw.
         dump_html(html_string)
         raise
     return tree
 
 
 def find_table_by_thead_content(tree, table_head_contents):