build system wants to install test directory

[mschubert]

I'm getting the following error when using the distutils-based installation on gentoo:

 * ERROR: dev-python/reretry-0.11.2::mschubert failed (install phase):
 *   Package installs 'tests' package which is forbidden and likely a bug in the build system.

# ls -la image/home/m.schubert/gentoo/usr/lib/python3.10/site-packages/tests/
total 16K
-rw-r--r-- 1 m.schubert m.schubert    0 Nov  1 14:09 __init__.py
drwxr-xr-x 2 m.schubert m.schubert 4.0K Nov  1 14:09 __pycache__/
-rw-r--r-- 1 m.schubert m.schubert 1.7K Nov  1 14:09 test_async.py
-rw-r--r-- 1 m.schubert m.schubert 4.1K Nov  1 14:09 test_retry.py

Related: hetznercloud/hcloud-python#40