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.
else:
            if css and self.stylesheets:
                css_attachments = self.css() or []
                for attachment in css_attachments:
                    attr = OrderedDict([
                        ["type", "text/css"],
                        ["rel", "stylesheet"],
                        ["href", attachment.url],
                    ])
                    response.append(("link", attr, None))
                if self.css_errors:
                    msg = '\n'.join(self.css_errors)
                    response.append(JavascriptAsset(self, inline=self.dialog_message(msg)).to_node())
            if js and self.javascripts:
                attr = OrderedDict([
                    ["async", "async" if async_load else None],
                    ["type", "text/javascript"],
                    ["src", self.js().url],
                ])
                response.append(("script", attr, None))
        return response
    @func.lazy_property
    def last_modified(self):
        """Returns last modified date of linked files"""
        return max(itertools.chain(
            (asset.last_modified for asset in self.javascripts),
            (asset.last_modified for asset in self.stylesheets),
        ))
    @func.lazy_property
    def version(self):