1. I don't think there is, this seems pretty good for forcing people's hand.
2. I think the error message is worded well, maybe a doc link, and maybe a link to the Streamlink issue? I don't know if that's going too far.

I think the message is fine without any links.

I already mentioned earlier that I want to move most of the metadata defined imperatively in setup.py into the declarative setup.cfg format and clean up everything. The package metadata is a bit messy right now and hasn't been touched in ages. There were a lot of changes in the past years in regards to python packaging standards, and we should stay up2date. It's a bit difficult though following and understanding all the changes here while having the supported python versions, dependencies and downstream system packages in mind.

After looking at the setuptools keywords docs, I found the following about the test_suite keyword, which is relevant for this PR/issue:

test_suite
A string naming a unittest.TestCase subclass (or a package or module containing one or more of them, or a method of such a subclass), or naming a function that can be called with no arguments and returns a unittest.TestSuite. If the named suite is a module, and the module has an additional_tests() function, it is called and the results are added to the tests to be run. If the named suite is a package, any submodules and subpackages are recursively added to the overall test suite.

Specifying this argument enables use of the test command to run the specified test suite, e.g. via setup.py test. See the section on the test command below for more details.

New in 41.5.0: Deprecated the test command.

Should we remove it in this PR, or should we do this when cleaning up the package metadata? Removing it instead of adding the sys.exit call doesn't have any effect though, despite what the documentation is saying. Either way, it should be removed at some point.

Should we remove it in this PR, or should we do this when cleaning up the package metadata?

Doing it in the clean up PR makes more sense. I think this is fine so I'm going to merge it in now.