Test regressions in 2.7.0

[mweinelt]

I bisected the following errors to 2744589.

FAILED pretix_sepadebit/tests/test_payment.py::test_mail_context - KeyError: 'due_date'
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder0] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder1] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder2] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder3] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder4] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder5] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder6] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder7] - AssertionError: assert False
FAILED pretix_sepadebit/tests/test_payment.py::test_call_mail_context[placeholder8] - AssertionError: assert False