Include all files from jupyter_core

Setuptools is not including `juptyer_core/*py` files.

setup.cfg

@@ -20,13 +20,18 @@ classifiers      =
 
 [options]
 py_modules = jupyter
-packages = jupyter_core, jupyter_core.utils, jupyter_core.tests
+packages = find:
 include_package_data = True
 python_requires = >=3.6
 install_requires =
     traitlets
     pywin32>=1.0 ; sys_platform == 'win32' and platform_python_implementation != 'PyPy'
 
+[options.packages.find]
+exclude =
+    examples
+    jupyter_core.tests
+
 [options.entry_points]
 console_scripts =
     jupyter              = jupyter_core.command:main