CI failures on Python 3.9 since scheduled run on 2026-02-21

[ajalon1]

CI for Python 3.9 has been failing since this scheduled run on 2026-02-21. There are two separate issues:

1. pkg_resources missing in minimal version tests

The test (3.9, ubuntu-latest, minimal) job fails with:

ModuleNotFoundError: No module named 'pkg_resources'

This happens because the minimal pinned version of mkdocs (1.1.1) imports pkg_resources, which was removed from setuptools in newer versions and is no longer installed by default on recent Python/pip.

2. virtualenv 21.0.0 broke hatch on Python 3.9

As of virtualenv 21.0.0 (released 2026-02-25), all Python 3.9 CI jobs fail with:

Environment `test` is incompatible: module 'virtualenv.discovery.builtin' has no attribute 'propose_interpreters'

This is a known upstream issue: pypa/hatch#2193

Fix

The virtualenv issue is addressed in #78 by pinning virtualenv<21 for Python 3.9 jobs. The pkg_resources issue in the minimal test may need a bump of the minimum mkdocs version or adding setuptools as a test dependency.