actually test different python versions (#7242)

Author: keewis

.github/workflows/ci.yaml

@@ -92,6 +92,8 @@ jobs:
           environment-name: xarray-tests
           cache-env: true
           cache-env-key: "${{runner.os}}-${{runner.arch}}-py${{matrix.python-version}}-${{env.TODAY}}-${{hashFiles(env.CONDA_ENV_FILE)}}"
+          extra-specs: |
+            python=${{matrix.python-version}}
 
       # We only want to install this on one run, because otherwise we'll have
       # duplicate annotations.