Add type marker for PEP 561 (#70)

This indicates to type checker tools that typesystem has inline
type hints.

More information on this can be found at https://www.python.org/dev/peps/pep-0561/#id18

Author: trickeydan

setup.py

@@ -39,6 +39,7 @@ def get_packages(package):
     author='Tom Christie',
     author_email='tom@tomchristie.com',
     packages=get_packages('typesystem'),
+    package_data={'typesystem': ['py.typed']},
     install_requires=[],
     python_requires='>=3.6',
     classifiers=[
@@ -50,5 +51,6 @@ def get_packages(package):
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
-    ]
+    ],
+    zip_safe=False
 )