Minimum Python version is actually 3.9

We use 3.9-style type annotations in this codebase. To support 3.8 in the future, we'll need to use a compatible style type annotation.

Fixes #48.

PiperOrigin-RevId: 571504993

setup.py

@@ -50,7 +50,7 @@
     },
     url='https://github.com/google/xee',
     packages=setuptools.find_packages(exclude=['examples']),
-    python_requires='>=3.8',
+    python_requires='>=3.9',
     entry_points={
         'xarray.backends': ['ee=xee:EarthEngineBackendEntrypoint'],
     }