ci: move Cygwin tests to Python 3.12

The Cygwin test jobs fail during python3.9 -m venv because the generic python-pip-wheel package advanced from 26.0.1 to 26.2.1, which requires Python 3.10 or newer. Both fast and performance jobs fail in ensurepip before reaching the test suite.

Use the maintained Cygwin python312 package and executable instead. This preserves Cygwin platform coverage while the regular matrix continues covering Python 3.9.

Validation: the workflow parses as YAML, and the current Cygwin python312 package contains python3.12.exe and ensurepip.

Author: codex

.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: git python312 python-pip-wheel python-setuptools-wheel python-wheel-wheel
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
@@ -73,7 +73,7 @@ jobs:
 
     - name: Set up virtual environment
       run: |
-        python3.9 -m venv .venv
+        python3.12 -m venv .venv
         echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"
 
     - name: Update PyPA packages