Django 6.0: `django.core.mail.BadHeaderError` -> `ValueError`

[adamchainz]

Description

Swap BadHeaderError to ValueError and remove it from the import, per:

https://docs.djangoproject.com/en/6.0/releases/6.0/#default-auto-field-setting-now-defaults-to-bigautofield:~:text=The%20django%2Ecore%2Email%2EBadHeaderError%20exception%20is%20deprecated

https://github.com/django/django/blob/dba622ebc1f6a6700b75303a65f8a334bd46bd8e/django/core/mail/message.py#L46