Opt-in linkahead package to mypy type checking

See https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages for how to opt-in to mypy type checking in user code.

setup.py

@@ -193,7 +193,7 @@ def setup_package():
         tests_require=["pytest", "pytest-cov", "coverage>=4.4.2",
                        "jsonschema>=4.4.0"],
         package_data={
-            'linkahead': ['cert/indiscale.ca.crt', 'schema-pycaosdb-ini.yml'],
+            'linkahead': ['py.typed', 'cert/indiscale.ca.crt', 'schema-pycaosdb-ini.yml'],
         },
         scripts=[
             "src/linkahead/utils/caosdb_admin.py",