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We had to make essentially the same changes to test matrix files for tox and Travis in dozens of repositories; we have since automated much of that for future Python, Django, and Django REST Framework upgrades.
Django 2.0 removed support for bytestrings from some APIs that had previously accepted them mainly in order to support Python 2. Our tests missed many of these, because they aren’t always consistent with the actual code in the type of data provided as arguments. Type annotations may have caught this much earlier, even if we just checked against the ones already provided for Django itself.
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