web/web_widget_datepicker_options/view/qweb.xml

<odoo>
    <template id="assets_backend" name="web_widget_datepicker_options assets" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <script type="text/javascript" src="/web_widget_datepicker_options/static/src/js/datepicker.js"></script>
        </xpath>
    </template>
</odoo>