Add windows compatibility for tests

[collerek]

Add windows support for tests:

• different path to venv scripts
• aiopg needs a workaround with event loop setting on py3.8+

Should be reviewed after #453 is merged to highlight only relevant changes

[aminalaee]

Let's wait for the base branch to be dealt with so we can review this. But I like the windows compatibility approach anyway.

Maybe we can also add it to the GitHub Actions.