Use sqlite instead of postgres for tests

[rpkilby]

Just a minor change to use sqlite instead of postgres.

• It wasn't clear to me if there was a reason postgres was used
• Sqlite is a little easier to get started with in development

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Edit: Also added the flake8 build to CI.

[Bogdanp]

Thanks! The reason this used pg to begin with is that, initially, the model used some pg-specific columns.

I merged your changes with rebase here: https://github.com/Bogdanp/django_dramatiq/compare/baf783e97bd6287d60a26cf509993f10b3466995...8479cb2