ci: restore Cygwin virtual environments on Python 3.9

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Cygwin patches ensurepip to load setuptools and pip wheels from
`/usr/share/python-wheels`. The rolling python-pip-wheel package advanced to pip
26.2.1, which requires Python 3.10 or newer, so every Python 3.9 venv creation
failed—including the installation test's nested virtual environment.

Keep the single Cygwin lane on Python 3.9 because the regular matrix already
covers Python 3.12 and newer. Install Cygwin's setuptools wheel, download pip
26.0.1 from its immutable PyPI URL into ensurepip's shared wheel directory,
verify its SHA-256 digest, and avoid upgrading pip. This fixes all venv creation
without relying on a version that may disappear from rolling Cygwin mirrors.

Assisted-by: GPT 5.6
Co-authored-by: GPT 5.6 <codex@openai.com>

Author: Byron

.github/workflows/cygwin-test.yml

@@ -45,7 +45,7 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v6
       with:
-        packages: git python39 python-pip-wheel python-setuptools-wheel python-wheel-wheel
+        packages: curl git python39 python-setuptools-wheel
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
@@ -73,12 +73,15 @@ jobs:
 
     - name: Set up virtual environment
       run: |
+        pip_wheel=/usr/share/python-wheels/pip-26.0.1-py3-none-any.whl
+        curl -fsSLo "$pip_wheel" https://files.pythonhosted.org/packages/de/f0/c81e05b613866b76d2d1066490adf1a3dbc4ee9d9c839961c3fc8a6997af/pip-26.0.1-py3-none-any.whl
+        python3.9 -c 'import hashlib, pathlib, sys; assert hashlib.sha256(pathlib.Path(sys.argv[1]).read_bytes()).hexdigest() == sys.argv[2]' "$pip_wheel" bdb1b08f4274833d62c1aa29e20907365a2ceb950410df15fc9521bad440122b
         python3.9 -m venv .venv
         echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"
 
     - name: Update PyPA packages
       run: |
-        python -m pip install -U pip 'setuptools; python_version<"3.12"' wheel
+        python -m pip install -U 'setuptools; python_version<"3.12"' wheel
 
     - name: Install project and test dependencies
       run: |