www/py-django-mptt: Update to 0.11.0

• Also assign the port to Django 2.2 because Django 1.11 is End-of-Life since April.

• Remove superfluous USES=gettext because gettext is already pulled in by Python itself (via the NLS option). Although the package contains some localized .po files, Django needs Python's gettext API to work with those files.

• Clean up CONFLICTS_INSTALL as www/py-dj21-django-mptt no longer exists in the Ports tree.

Changelog:

https://github.com/django-mptt/django-mptt/compare/0.9.1...0.11.0

PR: 245472
Approved by: maintainer timeout (14 days)