Django-upgrade

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* Add Django 3.2+ fixer to remove ``default_app_config`` assignments in ``__init__.py`` files.

Thanks to Bruno Alla in `PR 140 <https://github.com/adamchainz/django-upgrade/pull/140>`__.

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* Fix URL rewriting to avoid converting regular expressions that don’t end with ``$``.
If the ``$`` is missing, Django will search for the given regular expression anywhere in the path.

Thanks to qdufrois for the report in `Issue 121 <https://github.com/adamchainz/django-upgrade/issues/121>`__.

* Made ``JSONField`` and ``NullBooleanField`` fixers ignore migrations files.
Django kept these old field classes around for use in historical migrations, so there’s no need to rewrite such cases.

Thanks to Matthieu Rigal and Bruno Alla for the report in `Issue 79 <https://github.com/adamchainz/django-upgrade/issues/79>`__.

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* Add Django 2.0+ fixer to rewrite imports of ``lru_cache`` from ``django.utils.functional`` to use ``functools``.

* Support Django 4.0 as a target version.
There are no fixers for it at current.
Most of its deprecations don’t seem automatically fixable.

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* Avoid rewriting ``request.META`` to ``request.headers`` in assignments.
This pattern is used in tests, and works for ``request.META`` but not ``request.headers``.

Thanks to Bruno Alla for the report in `Issue 74 <https://github.com/adamchainz/django-upgrade/issues/74>`__.

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* Fix import fixers to not crash on star imports (``from foo import *``).

Thanks to Mikhail for the report in `Issue 70 <https://github.com/adamchainz/django-upgrade/issues/70>`__.

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* Fix ``get_random_string()`` fixer to not add the argument to calls like ``crypto.get_random_string(12)``.

* Add fixers to remove various compatibility imports removed in Django 3.1.

Thanks to Bruno Alla in `PR 44 <https://github.com/adamchainz/django-upgrade/pull/44>`__.

* Add fixer for Django 2.2 to rewrite ``request.META`` access of headers to ``HttpRequest.headers``.

* Add fixer for Django 2.0 to rewrite ``include()`` and ``url()`` from ``django.conf.urls`` to ``django.urls``.
``url()`` may be rewritten to ``path()`` or ``re_path()`` accordingly.

Thanks to Bruno Alla for the original implementation of regex-to-path conversion in django-codemod.
Thanks to Matthias Kestenholz for an initial PR.

* Add fixer for Django 1.9 requirement to pass ``on_delete`` to ``ForeignKey`` and ``OneToOneField``.

Thanks to Bruno Alla in `PR 61 <https://github.com/adamchainz/django-upgrade/pull/61>`__.