Use `python-version` in setup-python action

Before this change, a deprecated `version` arg was used.

.github/workflows/python-tests.yml

@@ -32,7 +32,7 @@ jobs:
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v1
       with:
-        version: ${{ matrix.python-version }}
+        python-version: ${{ matrix.python-version }}
     - name: Upgrade pip/setuptools/wheel
       run: >-
         python