Fix YAML syntax for Python version exclusion

Author: pgleeson

.github/workflows/ci_versions.yml

@@ -19,7 +19,7 @@ jobs:
         runs-on: [ubuntu-latest, macos-latest ]
         exclude:
           - runs-on: macos-latest
-            python-version: ["3.9"]
+            python-version: "3.9"
 
     steps:
     - uses: actions/checkout@v4