setup.py installs the tests package about tomlkit

[jtrakk]

tomlkit's tests directory caused a collision in nixpkgs. As I said in my Poetry PR,

If multiple distributions have tests as a top-level package, they'll conflict whenever both are installed. (Examples here and here). Two common alternative strategies are:

1. not distributing tests (as here), or
2. placing tests in a subdirectory of the main package, rather than adjacent (as here and here).

Either of these strategies will avoid this issue.

[wgjak47]

This problem also happen in gentoo.

 * ERROR: dev-python/tomlkit-0.7.0::gentoo-zh failed (install phase):
 *   Package installs 'tests' package which is forbidden and likely a bug in the build system.
 * 
 * Call stack:
 *     ebuild.sh, line  125:  Called src_install
 *   environment, line 2962:  Called distutils-r1_src_install
 *   environment, line 1187:  Called _distutils-r1_run_foreach_impl 'distutils-r1_python_install'
 *   environment, line  439:  Called python_foreach_impl 'distutils-r1_run_phase' 'distutils-r1_python_install'
 *   environment, line 2576:  Called multibuild_foreach_variant '_python_multibuild_wrapper' 'distutils-r1_run_phase' 'distutils-r1_python_install'
 *   environment, line 2058:  Called _multibuild_run '_python_multibuild_wrapper' 'distutils-r1_run_phase' 'distutils-r1_python_install'
 *   environment, line 2056:  Called _python_multibuild_wrapper 'distutils-r1_run_phase' 'distutils-r1_python_install'
 *   environment, line  778:  Called distutils-r1_run_phase 'distutils-r1_python_install'
 *   environment, line 1155:  Called distutils-r1_python_install
 *   environment, line 1074:  Called die
 * The specific snippet of code:
 *               die "Package installs '${p}' package which is forbidden and likely a bug in the build system.";
 * 
 * If you need support, post the output of `emerge --info '=dev-python/tomlkit-0.7.0::gentoo-zh'`,
 * the complete build log and the output of `emerge -pqv '=dev-python/tomlkit-0.7.0::gentoo-zh'`.
 * The complete build log is located at '/var/tmp/portage/dev-python/tomlkit-0.7.0/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-python/tomlkit-0.7.0/temp/environment'.
 * Working directory: '/var/tmp/portage/dev-python/tomlkit-0.7.0/work/tomlkit-0.7.0'
 * S: '/var/tmp/portage/dev-python/tomlkit-0.7.0/work/tomlkit-0.7.0'

[bollwyvl]

Also conda-forge: conda-forge/tomlkit-feedstock#24

We'd love the tests to be present in the sdist (a la recursive-include tests * and include_package_data=True) so we can run them, but excluded (a la find_packages(".", exclude=["tests*"]) so they don't cause this conflict. Alas, I do not know the poetry constructs for these techniques.

[bollwyvl]

Perhaps:

[tool.poetry]
# ...
include = ["tests"]

and omit the line about including the tests in the sdist. I presume the whl will then have them, too, but that's another issue for another day, i suppose...

[kwshi]

Relevant issue from Poetry: python-poetry/poetry#2353