Edit demo.py for pytest

[eggplants]

To make testing more easily and measure coverage with pytest-cov, we have to edit demo.py for pytest.

[eggplants]

I would appreciate it if someone could work on this instead of me. Sorry...

[nautics889]

I see there are harcoded ID's of Pixiv users, illustrations, etc in demo.py. I'd rather use mock responses from Pixiv for unittest coverage instead of requesting the original Pixiv resource each time while running unittests (pytest).

For now i suggest to create a separate tests.py module and put there pytest functions operating with fixtures and mocked requests. @eggplants, what you think?