Migrate tox.ini to use extras = [] instead of deps = -r requirements-foo.txt or deps = .[bar]

To be able to migrate away from requirements files to setup.cfg, we should make our tox configuration not use them.

This is the opportunity to move to the extras option, which is the built-in way of doing deps = .[foo] (which also avoids installing the package from . instead of from the sdist generated by tox).

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Migrated from T2107 (view on Phabricator)