🐛 Fix installing by calling find_namespace_packages in setup.py

Turns out just removing __init__.py is not enough for native namespace
packages. We also need to (explicitly) call setuptools.find_namespace_packages()
for setup.py to find the package...

https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

Fixes gh-77.

setup.py

@@ -1,5 +1,5 @@
 from io import open
-from setuptools import find_packages, setup
+from setuptools import find_namespace_packages, find_packages, setup
 
 with open("requirements.txt") as fp:
     install_requires = fp.read()
@@ -16,7 +16,8 @@ setup(
     long_description_content_type="text/markdown",
     keywords="qurator ocr",
     license="Apache",
-    packages=find_packages(exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
+    packages=find_namespace_packages(include=["qurator.*"])
+    + find_packages(exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
     install_requires=install_requires,
     tests_require=tests_require,
     package_data={