www/py-django-post_office: Update to 3.11

• Switch to the PEP517 build framework.

• Remove no longer required USE_LOCALE as the UTF-8 mode is enabled by default when the locale is C since Python 3.7.

Changelog since 3.9.1:

https://github.com/ui/django-post_office/blob/v3.11/CHANGELOG.md

PR: 292602
Approved by: maintimer timeout (14+ days)