[BuildStream] Dropping support for `setup.py test`

[Chandan Singh <chandan chandansingh net>]

Hi all,

In this thread, I am proposing to remove support for running tests via
`setup.py test`. More details following, but please respond if you think this
will negatively affect some workflows.

Currently, BuildStream recommends using `tox` as the primary test runner, but we
also support `setup.py test` to some extent. In practice, it is unverified and
does not offer a good developer experience for following reasons:

* `setup.py test` does not handle build requirements correctly, so it will _not_
  install Cython automatically before running tests.

* External plugins are currently not listed as hard dependency, so running any
  tests that involves external plugins will also fail if the user doesn't happen
  to have the right version installed.

* Last but not the least, it is officially deprecated [1]. Setuptools folks
  recommend using `tox` in similar cases, which we already do.

Personally, I am not sure what problem we are solving with supporting
`setup.py test`. Quoting from our documentation:

> While using tox is practical for developers running tests in more predictable
> execution environments, it is still possible to execute the test suite against
> a specific installation environment using pytest directly:
>
> ./setup.py test

In this case, I am not sure what `setup.py test` offers on top of just running
`pytest`. If the user has to install Cython, BuildStream and external plugins
manually anyway, why not let them deal with test dependencies as well? Also,
`tox --sitepackages` should also be equivalent in these scenarios.

`setup.py test` is going to be removed in a future setuptools version anyway. Do
people see value in supporting it in the meantime?

Thanks,
Chandan

[1]: https://setuptools.readthedocs.io/en/latest/setuptools.html#test-build-package-and-run-a-unittest-suite