add py.typed marker file for PEP-561 support (#391)

Fix #390

Disable zip_safe option as mypy is incompatible with it.

setup.py

@@ -68,8 +68,9 @@ setup(
     url='https://github.com/samuelcolvin/pydantic',
     license='MIT',
     packages=['pydantic'],
+    package_data={'pydantic': ['py.typed']},
     python_requires='>=3.6',
-    zip_safe=True,
+    zip_safe=False,  # https://mypy.readthedocs.io/en/latest/installed_packages.html
     install_requires=[
         'dataclasses>=0.6;python_version<"3.7"'
     ],