Update django-test-plus to 1.0.13

[pyup-bot]

There's a new version of django-test-plus available.
You are currently using 1.0.11. I have updated it to 1.0.13

Changelog

1.0.13

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• Added response_401() test
• Fixed situation where User models without a 'username' field could not be
  used as easily. Now credential field is automatically determined.
• Fixed assertLoginRequired when settings.LOGIN_URL is a named URL pattern
• Removed support for Django 1.4.x as it is well beyond it's end of life and causes a headache for supporting newer releases

1.0.12

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• Fixed incorrect documentation
• Added response_405 and response_410 test methods

Happy merging! 🤖