missing python_requires: v2.28 introduces breaking change for python 3.9 #860

Hi 👋

This PR introduces a call to packages_distributions:

This function was introduced in python 3.10 (docs).

This is a breaking change for python 3.9 installing google-api-core v2.28.*:

An error occurred: module 'importlib.metadata' has no attribute 'packages_distributions'

Can you bump python_requires to >=3.10 and yank all existing 2.28.* versions from PyPI?

Until yanking is complete, I'm adding "google-api-core<2.28;python_version<'3.10" to our requirements.

Example CI failure: https://github.com/piskvorky/smart_open/actions/runs/18954411130/job/54127375199

Originally posted by @ddelange in #832 (comment)