update syntax

Author: parthea

.github/workflows/tests.yaml

@@ -39,7 +39,7 @@ 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: ${ALL_PYTHON}
+        python: ${{ fromJSON(ALL_PYTHON) }}
         exclude:
           # Remove once https://github.com/googleapis/gapic-generator-python/issues/2303 is fixed
           - python: 3.7
@@ -60,7 +60,7 @@ jobs:
       # Run showcase tests on the lowest and highest supported runtimes
       matrix:
         # TODO(https://github.com/googleapis/gapic-generator-python/issues/2121) Remove `showcase_w_rest_async` target when async rest is GA.
-        python: ["${ALL_PYTHON}[0]", "${ALL_PYTHON}[-1]"]
+        python: ["${{ fromJSON(ALL_PYTHON) }}[0]", "${{ fromJSON(ALL_PYTHON) }}[-1]"]
         target: [showcase, showcase_w_rest_async]
         logging_scope: ["", "google"]
 
@@ -151,7 +151,7 @@ jobs:
   showcase-unit:
     strategy:
       matrix:
-        python: ${ALL_PYTHON}
+        python: ${{ fromJSON(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"]