ci: retain Python 3.9 for Cygwin coverage

The Cygwin workflow has only one interpreter entry, and Python 3.9 was intentionally retained as the most relevant packaged version when that job was established. Other workflows already cover newer Python releases.

Restore the existing Cygwin and Python 3.9 combination now that the actual pip wheel regression is handled independently. The pinned pip 26.0.1 bootstrap supports Python 3.9.

Validated with git diff --check, Ruby YAML parsing, and PyPI metadata showing that pip 26.0.1 requires Python 3.9 or newer.

Author: codex

.github/workflows/cygwin-test.yml

@@ -45,7 +45,7 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v6
       with:
-        packages: curl git python312
+        packages: curl git python39
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
@@ -74,9 +74,9 @@ jobs:
     - name: Set up virtual environment
       run: |
         pip_wheel=/tmp/pip-26.0.1-py3-none-any.whl
-        python3.12 -m venv --without-pip .venv
+        python3.9 -m venv --without-pip .venv
         curl -fsSLo "$pip_wheel" https://files.pythonhosted.org/packages/de/f0/c81e05b613866b76d2d1066490adf1a3dbc4ee9d9c839961c3fc8a6997af/pip-26.0.1-py3-none-any.whl
-        python3.12 -c 'import hashlib, pathlib, sys; assert hashlib.sha256(pathlib.Path(sys.argv[1]).read_bytes()).hexdigest() == sys.argv[2]' "$pip_wheel" bdb1b08f4274833d62c1aa29e20907365a2ceb950410df15fc9521bad440122b
+        python3.9 -c 'import hashlib, pathlib, sys; assert hashlib.sha256(pathlib.Path(sys.argv[1]).read_bytes()).hexdigest() == sys.argv[2]' "$pip_wheel" bdb1b08f4274833d62c1aa29e20907365a2ceb950410df15fc9521bad440122b
         PYTHONPATH="$pip_wheel" .venv/bin/python -m pip install "$pip_wheel"
         echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"