Resolve warnings from modern flake8

src/pretix/base/services/export.py

@@ -22,4 +22,6 @@ if settings.HAS_CELERY:
     from pretix.celery import app
 
     export_task = app.task(export)
-    export = lambda *args, **kwargs: export_task.apply_async(args=args, kwargs=kwargs)
+
+    def export(*args, **kwargs):
+        export_task.apply_async(args=args, kwargs=kwargs)

src/pretix/base/services/mail.py

@@ -90,4 +90,6 @@ if settings.HAS_CELERY and settings.EMAIL_BACKEND != 'django.core.mail.outbox':
     from pretix.celery import app
 
     mail_send_task = app.task(mail_send)
-    mail_send = lambda *args, **kwargs: mail_send_task.apply_async(args=args, kwargs=kwargs)
+
+    def mail_send(*args, **kwargs):
+        mail_send_task.apply_async(args=args, kwargs=kwargs)

src/pretix/base/services/tickets.py

@@ -32,4 +32,6 @@ if settings.HAS_CELERY:
     from pretix.celery import app
 
     generate_task = app.task(generate)
-    generate = lambda *args, **kwargs: generate_task.apply_async(args=args, kwargs=kwargs)
+
+    def generate(*args, **kwargs):
+        generate_task.apply_async(args=args, kwargs=kwargs)

src/pretix/control/templatetags/order_overview.py

@@ -8,12 +8,15 @@ register = template.Library()
 
 @register.filter(name='togglesum', needs_autoescape=True)
 def cut(value, autoescape=True):
+    def noop(x):
+        return x
+
     if not value:
         return ''
     if autoescape:
         esc = conditional_escape
     else:
-        esc = lambda x: x
+        esc = noop
     return mark_safe('<span class="count">{0}</span><span class="sum">{1}</span>'.format(
         esc(value[0]), esc(formats.localize(value[1]))
     ))