build(deps-dev): update django-mptt requirement from ~=0.14.0 to ~=0.15.0

[dependabot]

Updates the requirements on django-mptt to permit the latest version.

Changelog

Sourced from django-mptt's changelog.

========== Change log

Next version

0.15

• Since I unfortunately still depend on django-mptt in prehistoric projects I took it upon me to make it runnable again. This doesn't mean I want to maintain the package.
• Added Django 4.2.
• Dropped Python < 3.9, Django < 3.2.
• Started using ruff and more rules.
• Dropped the index_together check on Django 5 and better. Django 5 only supports Meta.indexes.
• Optimized tree rebuilding.
• Added support for the Django admin theme switcher.
• Switched to hatchling.

0.14

• Merged the docs/upgrade.rst file into the main CHANGELOG.rst.
• Fixed the Sphinx autodoc configuration to also work locally. Ensured that readthedocs is able to build the docs again.
• Fixed a bug where DraggableMPTTAdmin assumed that the user model's primary key is called id.
• Ensured that we do not install the tests.myapp package.
• Added dark mode support to the draggable model admin.
• Started using pre-commit, for the fun of it.

0.13

• MARKED THE PROJECT AS UNMAINTAINED, WHICH IT STILL IS
• Reformatted everything using black, isort etc.
• Switched from Travis CI to GitHub actions.
• Switched to a declarative setup.
• Verified compatibility with Django up to 3.2 and Python up to 3.9. Dropped compatibility guarantees (ha!) with anything older than Django 2.2 and Python 3.6.
• Renamed the main development branch to main.
• Fixed a problem where our _get_user_field_names mistakenly returned abstract fields.

... (truncated)

Commits

• 23cd837 django-mptt 0.15
• 4cc428e Remove Django 4.1 from CI
• f7999b7 Bail out
• a5827a8 Test compatibility
• af6072d Maybe
• 02b7ee3 Annoying
• 6df312f Hmm
• b717e4d lol
• d60e7b9 Switch to hatch, missed another index_together occurrence
• c169d90 Bye, index_together check if Django is too new
• Additional commits viewable in compare view

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[afuetterer]

django-mptt dropped support for Python 3.8. The dependency should be pinned instead.

"django-mptt==0.14.0"

Add the following to dependabot config.

ignore:
      - dependency-name: "django-mptt"

django-mptt dropped support for Python 3.8

[dependabot]

OK, I won't notify you again about this release, but will get in touch when a new version is available. If you'd rather skip all updates until the next major or minor version, let me know by commenting @dependabot ignore this major version or @dependabot ignore this minor version. You can also ignore all major, minor, or patch releases for a dependency by adding an ignore condition with the desired update_types to your config file.

If you change your mind, just re-open this PR and I'll resolve any conflicts on it.