Make setup.py example equivalent to setup.cfg in test_cli_compile

[abravalheri]

This is an internal/maintenance change to the test suite.

As pointed out in pypa/setuptools#3198 (comment), the tests in:

test_cli_compile.py::test_input_formats[setup.py]
test_cli_compile.py::test_multiple_extras[setup.py-singular]
test_cli_compile.py::test_one_extra[setup.py]
test_cli_compile.py::test_multiple_extras[setup.py-comma-separated]

are currently failing.

After checking the code, it seems that the setup.py file does not match exactly its setup.cfg counterpart and is actually trying to create an empty package. (See pypa/setuptools#3198 (comment)).

The changes introduced here make the setup.py mirror exactly its setup.cfg counterpart.
If instead the intention is to create purposefully empty packages, then setuptools v61 asks the users to explicitly set:

setup(
    ...,
    packages=[]
)

P.S.: I think this change can probably be a #skip-changelog. Please feel free to rename the PR.

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).