feat: add py.typed marker for mypy users (#332)

This lib is pretty well typed, but MyPy doesn't know to look for the
types by default. Adding a py.typed file will tell MyPy to look at the
type hints in the package directly. It's a pretty simple add for some
big gains!

https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages

Co-authored-by: fozcodes <[email protected]>

Author: fozcodes

schemachange/py.typed

@@ -40,3 +40,6 @@ dev =
     pre-commit
     pyfakefs
     pytest
+
+[options.package_data]
+schemachange = py.typed