Here is our complete solution. Thanks to Dr Obx and Axel.

1) Download jquery mask plugin

2) Install it in ourmodule/static/src/js

3) Load the script

        <template id="ourmodule.assets_frontend" inherit_id="website.assets_frontend" name="Shop">

            <xpath expr="." position="inside">

                <script type="text/javascript" src="/website_lapagept/static/src/js/website_sale_clear_cart.js"></script>

                <script type="text/javascript" src="/website_lapagept/static/src/js/jquery-mask-plugin-master/src/jquery.mask.js"></script>

            </xpath>

        </template>

4) Add this in the file ourmodule/static/src/js/jquery-mask-plugin-master/src/jquery.mask.js

$(document).ready(function(){

//  $('.zipcode').mask('A0A 0A0');

        $('.zipcode').mask('Y9Y 9Y9',

                           {'translation': {

                                        9: {pattern: /[0-9]/},  

                                        Y: {pattern: /[A-Z]/}

                                      }

                                });

});

5) Add the correct .zipcode class to the field that should have the mask

                  <div t-attf-class="form-group #{error.get('zip') and 'has-error' or ''} col-lg-6">

                      <label class="control-label" for="zip">Code postal</label>

                      <input type="text" name="zip" class="form-control zipcode" t-att-value="checkout.get('zip')"/>

                  </div>

AND ALSO

                  <div t-attf-class="form-group #{error.get('shipping_zip') and 'has-error' or ''} col-lg-6">

                      <label class="control-label" for="shipping_zip" style="font-weight: normal">Zip / Postal Code</label>

                      <input type="text" name="shipping_zip" class="form-control zipcode" t-att-value="checkout.get('shipping_zip', '')" t-att-readonly=" 'readonly' if shipping_id &gt;= 0 else ''"/>

                  </div>

And it works fine!