2.0.0 installs tests top-level

[dvzrv]

Hi! When packaging 2.0.0 for Arch Linux i noticed, that the tests are installed top-level.
This conflicts directly with django-classy-tags (which does the same wrong thing).

It is okay if the tests are being installed below the respective package's namespace, but never top-level, as this leads to conflicts:

/usr/lib/python3.8/site-packages/tests/__init__.py exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/__pycache__/__init__.cpython-38.opt-1.pyc exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/__pycache__/__init__.cpython-38.pyc exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/__pycache__/settings.cpython-38.opt-1.pyc exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/__pycache__/settings.cpython-38.pyc exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/__pycache__/test_core.cpython-38.opt-1.pyc exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/__pycache__/test_core.cpython-38.pyc exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/settings.py exists in both 'python-django-classy-tags' and 'python-django-sekizai'
/usr/lib/python3.8/site-packages/tests/test_core.py exists in both 'python-django-classy-tags' and 'python-django-sekizai'

[marksweb]

@dvzrv Sorry it's taken a while to pick this up. Just preparing for a v3 release.

Looks like I can avoid this by excluding the tests from packaging so going to raise a PR now.