Add default testpaths to pytest's config

[vytas7]

Some of our users/packagers have expressed a wish to be able to run pytest without any parameters. Otherwise pytest seems to pick all tests, including tutorials and what not.

Add testpaths = tests to our configs.

While at it, maybe we should consolidate these configs inside pyproject.toml, as opposed to setup.cfg?
The pytest docs state that

Usage of setup.cfg is not recommended unless for very simple use cases. .cfg files use a different parser than pytest.ini and tox.ini which might cause hard to track down problems. When possible, it is recommended to use the latter files, or pyproject.toml, to hold your pytest configuration.