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.
"""
            Checks if the bundle contains any sass/less content, then compiles it to css.
            Returns the bundle's flat css.
        """
        for atype in (SassStylesheetAsset, LessStylesheetAsset):
            assets = [asset for asset in self.stylesheets if isinstance(asset, atype)]
            if assets:
                cmd = assets[0].get_command()
                source = '\n'.join([asset.get_source() for asset in assets])
                compiled = self.compile_css(cmd, source)
                if not self.css_errors and old_attachments:
                    old_attachments.unlink()
                fragments = self.rx_css_split.split(compiled)
                at_rules = fragments.pop(0)
                if at_rules: