Django-capture-on-commit-callbacks

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* Add type hints.

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* Move incompatibility check for Django 3.2 from a system check to a use time
check. This is because pytest does not run system checks.

* Stop distributing tests to reduce package size. Tests are not intended to be
run outside of the tox setup in the repository. Repackagers can use GitHub's
tarballs per tag.

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* Support Django 3.2.

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* Drop Python 3.5 support.
* Support Python 3.9.

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* Drop Django 2.0 and 2.1 support.
* Use ``contextlib.contextmanager``.

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* Made ``captureOnCommitCallbacks()`` a ``classmethod`` so it can be used from within class methods such as ``setUpClass()``, ``setUpTestData()``.
* Avoiding capturing callbacks enqueued within rolled back ``atomic()`` blocks.
As a side effect of this change, the returned list of callbacks is only populated when the context manager exits.
* Add Django 3.1 support.