provide a test environment with missing dependencies

[kmike]

This should allow to drop some of the mock usages, and test it for real that the package works. I think a single environment without all the dependencies could work; it would only support dict items.

[elacuesta]

Sounds fine, although I don't think it should be a blocker for https://github.com/scrapy/scrapy/pull/3881.

One thing to consider is the coverage report. Currently those mocks allow to reach every line in a single environment. With this approach I'll need to do a few adjustments to combine reports from different tox envs. I'm pasting this here so I don't forget: https://help.github.com/en/actions/configuring-and-managing-workflows/persisting-workflow-data-using-artifacts

[kmike]

@elacuesta I think if you use codecov they combine reports automatically, on their side.