Select proper default for output_field based on distinct value

[javiplx]

Will return a set when distinct is enabled, and a list otherwise

[javiplx]

I believe that all functional tests succeed, but have no idea neither why the py38-multilint complains nor how to fix that particular error.

[adamchainz]

The error:

py38-codestyle run-test: commands[0] | multilint
Running black check
would reformat /home/runner/work/django-mysql/django-mysql/src/django_mysql/models/aggregates.py
Oh no! 💥 💔 💥
1 file would be reformatted, 90 files would be left unchanged.
black failed

Run black on the file to fix that.

[adamchainz]

So thinking about how we release this - it's a small breaking change. I think it will need a major version bump to communicate this, so I'd like to batch it with some other breaking changes I've not got around to making.

[adamchainz]

@javiplx It'll be faster if you run isort and black locally, rather than iterating against them on CI :) If you don't have python 3.8 locally, you should be able to run them with 3.6 or 3.7 just fine.

[javiplx]

Finally !!!!
Regarding releasing, I agree with the mayor approach because this is not backwards compatible. In any case we'll make an internal release based on 3.2.0, because we're still running django 1.11