According document of 'What’s New In Python 3.7', 'async' are reserved keywords now.

Code in line 105 of 'odoo/addons/base/ir/ir_qweb/assetsbundle.py' 
`def to_html(self, sep=None, css=True, js=True, debug=False, async=False, url_for=(lambda url: url)):`
will throw SyntaxError out if we launch odoo with this update python.
import base64
import os
import re
import hashlib
import itertools
import json
import textwrap
import uuid
from datetime import datetime
from subprocess import Popen, PIPE
from collections import OrderedDict
from odoo import fields, tools
from odoo.tools.pycompat import string_types, to_text
from odoo.http import request
from odoo.modules.module import get_resource_path
from odoo.addons.base.ir.ir_qweb.qweb import escape
import psycopg2
from odoo.tools import func, misc
import logging
_logger = logging.getLogger(__name__)
MAX_CSS_RULES = 4095
def rjsmin(script):