Django-ultracache

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. Django 4.0 compatibility.

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. Ensure cache coherency should a purger fail.

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. Django 3 compatibility.
. Fix potential thread local residual data issue.

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. Remove dependency on the sites framework everywhere. The sites framework is still automatically
considered if an installed app.
. Do not store metadata in the request anymore but in a list on thread locals.
. Introduce class utils.Ultracache to subject arbitrary pieces of Python code to caching.
. Drop Django 1 support.

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. Simpler class based decorator.
. Add Django 2.1 and Python 3.6 tests.

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. Add a test for tasks.