update syntax

Author: parthea

.github/workflows/tests.yaml

@@ -50,7 +50,7 @@ jobs:
       matrix:
         # Run mypy on all of the supported python versions listed in setup.py
         # https://github.com/python/mypy/blob/master/setup.py
-        python: ${{ fromJSON(needs.all_python_setup.outputs.all_python) }}
+        python: [${{ fromJSON(needs.all_python_setup.outputs.all_python) }}]
         exclude:
           # Remove once https://github.com/googleapis/gapic-generator-python/issues/2303 is fixed
           - python: 3.7
@@ -164,7 +164,7 @@ jobs:
     needs: all_python_setup
     strategy:
       matrix:
-        python: ${{ fromJSON(needs.all_python_setup.outputs.all_python) }}
+        python: [${{ fromJSON(needs.all_python_setup.outputs.all_python) }}]
         # TODO(https://github.com/googleapis/gapic-generator-python/issues/2121) Remove `_w_rest_async` variant when async rest is GA.
         variant: ['', _alternative_templates, _mixins, _alternative_templates_mixins, _w_rest_async]
         logging_scope: ["", "google"]