Move to Python 3.8, add tests

[adelavega]

WIP. Adding a set of comprehensive tests in Python 3.8.

Closes #6
Closes #7

[codecov-commenter]

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[adelavega]

@tyarkoni I added some basic tests for all the API routes. Looks like some routes (e.g. images datasets) aren't populated by ingest_neurosynth so can add tests for those later by creating fixtures, but I need to familiarize myself more with those models.

[tyarkoni]

@adelavega great, thanks! ingest_neurosynth shouldn't populate either of those models, so that's fine—images is for image-based data (Neurosynth is only coordinates), and datasets is a NIMADS constructions (it's basically a container for a set of annotated studies).

[adelavega]

Okay yep that's what I thought!