Specify test dependencies in pyproject.toml

[jokasimr]

Previously test dependencies had to be added both to basetest.in and to conda/meta.yaml.
That was a bit error prone. It's easy to forget updating conda test dependencies, and then things would crash next time the package was deployed.

This change makes test dependencies work similar to regular dependencies that we define in pyproject.yaml and then copy to meta.yaml and to base.in.

The advantage is we only need to write the test dependencies in one place.