Use find: in setup.cfg to find subpackages

`find:` docs at https://setuptools.readthedocs.io/en/latest/userguide/package_discovery.html

Refs #160

Include values from https://github.com/audreyfeldroy/cookiecutter-pypackage/blob/master/%7B%7Bcookiecutter.project_slug%7D%7D/setup.py#L60

Author: sverhoeven

tests/test_project.py

@@ -48,6 +48,6 @@ def test_subpackage(baked_withdevdeps):
     subpackage = (project_dir / 'my_python_package' / 'mysub')
     subpackage.mkdir()
     (subpackage / '__init__.py').write_text('FOO = "bar"', encoding="utf-8")
-    build_output = run('python3 setup.py build', project_dir)
+    build_output = run('env/bin/python3 setup.py build', project_dir)
     assert build_output.returncode == 0
     assert (project_dir / 'build' / 'lib' / 'my_python_package' / 'mysub' / '__init__.py').exists()

{{cookiecutter.project_name}}/setup.cfg

@@ -34,8 +34,7 @@ version = {{ cookiecutter.version }}
 [options]
 zip_safe = False
 include_package_data = True
-packages =
-    {{ cookiecutter.package_name }}
+packages = find:
 install_requires =
 
 [options.data_files]
@@ -57,10 +56,13 @@ dev =
     sphinx
     sphinx_rtd_theme
     recommonmark
-publishing = 
+publishing =
     twine
     wheel
 
+[options.packages.find]
+include = {{ cookiecutter.package_name }}, {{ cookiecutter.package_name }}*
+
 [coverage:run]
 branch = True
 source = {{ cookiecutter.package_name }}