feat: add py.typed to enable downstream type usage

Based on the instructions at
https://mypy.readthedocs.io/en/stable/installed_packages.html#making-pep-561-compatible-packages.

Author: gadomski

MANIFEST.in

@@ -0,0 +1 @@
+global-include *.typed

pystac_client/py.typed

@@ -28,7 +28,8 @@
     url="https://github.com/stac-utils/pystac-client.git",
     packages=find_packages(exclude=("tests",)),
     py_modules=[splitext(basename(path))[0] for path in glob("pystac_client/*.py")],
-    include_package_data=False,
+    include_package_data=True,
+    package_data={"pystac_client": ["py.typed"]},
     python_requires=">=3.7",
     install_requires=[
         "requests>=2.25",