Rename module async to tasks (#994)

Fixes #993

src/pretix/presale/views/async.py

@@ -1,3 +0,0 @@
-from pretix.base.views.async import *  # noqa
-
-# file kept for backwards compatibility

src/pretix/presale/views/cart.py

@@ -21,12 +21,12 @@ from pretix.base.models import (
 from pretix.base.services.cart import (
     CartError, add_items_to_cart, clear_cart, remove_cart_position,
 )
+from pretix.base.views.tasks import AsyncAction
 from pretix.multidomain.urlreverse import eventreverse
 from pretix.presale.views import (
     EventViewMixin, allow_cors_if_namespaced, allow_frame_if_namespaced,
     iframe_entry_view_wrapper,
 )
-from pretix.presale.views.async import AsyncAction
 from pretix.presale.views.event import (
     get_grouped_items, item_group_by_category,
 )

src/pretix/presale/views/order.py

@@ -30,11 +30,11 @@ from pretix.base.services.tickets import (
 )
 from pretix.base.signals import allow_ticket_download, register_ticket_outputs
 from pretix.base.views.mixins import OrderQuestionsViewMixin
+from pretix.base.views.tasks import AsyncAction
 from pretix.helpers.safedownload import check_token
 from pretix.multidomain.urlreverse import build_absolute_uri, eventreverse
 from pretix.presale.forms.checkout import InvoiceAddressForm, QuestionsForm
 from pretix.presale.views import CartMixin, EventViewMixin
-from pretix.presale.views.async import AsyncAction
 from pretix.presale.views.robots import NoSearchIndexViewMixin