add py.typed sentinal per https://www.python.org/dev/peps/pep-0561/#packaging-type-information

Author: ahupp

magic/py.typed

@@ -22,6 +22,7 @@ def read(file_name):
     long_description=read('README.md'),
     long_description_content_type='text/markdown',
     packages=['magic'],
+    package_data=['py.typed'],
     keywords="mime magic file",
     license="MIT",
     python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*',