CI: run the Windows diagnostic under bash so the heredoc works

The diagnostic step used Python heredoc syntax (`python - <<'PY' ... PY`)
but the step was running under pwsh, which does not understand <<.
PowerShell parse-errored on line 3 before anything ran, which is why the
second diagnostic run produced no pefile or WinDLL output at all.

Switch the step to `shell: bash` (Git Bash is installed by default on
GitHub Actions Windows runners) so the heredoc-into-python idiom works
unchanged. No other logic changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Technologicat

.github/workflows/ci.yml

@@ -79,7 +79,9 @@ jobs:
       # fails).
       - name: Diagnose extension DLL dependencies (Windows)
         if: runner.os == 'Windows'
-        shell: pwsh
+        # Use Git Bash so the Python heredoc works. PowerShell does not
+        # understand `python - <<'PY' ... PY` and chokes at parse time.
+        shell: bash
         run: |
           python -m pip install -q pefile
           python - <<'PY'