Fix UnicodeEncodeError when running unit tests in Windows

[stephenatgithub]

When running unit test in Windows, got this error:
UnicodeDecodeError: 'cp950' codec can't decode byte 0xc3 in position 268: illegal multibyte sequence

Besides Linux or MacOS, this is to provide an extra resolution for Windows.
https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/unit-tests/#many-test-failures-with-unicodeencodeerror

[github-actions]

Hello! Thank you for your contribution 💪

As it's your first contribution be sure to check out the patch review checklist.

If you're fixing a ticket from Trac make sure to set the "Has patch" flag and include a link to this PR in the ticket!

If you have any design or process questions then you can ask in the Django forum.

Welcome aboard ⛵️!

[carltongibson]

Can you set the PYTHONUTF8 environment variable, as per the Common Pitfalls section of the How to install Django on Windows
docs.

This should then pass. We don't support running with a non-utf-8 enconding.

I'm going to close this, assuming that setting the env var resolves your issue.

Thanks.