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