Add PEP-561 py.typed file

This commit adds a py.typed file, which marks the package as having type information available.
For more information, see  https://mypy.readthedocs.io/en/stable/installed_packages.html#making-pep-561-compatible-packages

Author: lalten

openhtf/py.typed

@@ -192,6 +192,7 @@ def run_tests(self):
     packages=find_packages(),
     package_data={
         'openhtf': [
+            'py.typed',
             'output/proto/*.proto', 'output/web_gui/dist/*.*',
             'output/web_gui/dist/css/*', 'output/web_gui/dist/js/*',
             'output/web_gui/dist/img/*', 'output/web_gui/*.*'