Explicitely list included packages

Setuptools doesn’t automatically find packages with no __init__.py file. We
have to manually list packages to avoid this limitation, for example for
ReadTheDocs installations.

setup.cfg

@@ -38,7 +38,13 @@ project_urls =
   Donation = https://www.patreon.com/kozea
 
 [options]
-packages = find:
+packages =
+  weasyprint
+  weasyprint.css
+  weasyprint.formatting_structure
+  weasyprint.layout
+  weasyprint.tests
+  weasyprint.tools
 zip_safe = false
 setup_requires = pytest-runner
 install_requires =