GitHub Actions: Add Python 3.10 to the testing (#1021)

https://pythoninsider.blogspot.com/2021/10/python-3100-is-available.html

Must be quoted in yaml... https://dev.to/hugovk/the-python-3-1-problem-85g

Author: cclauss

.github/workflows/tests.yml

@@ -6,12 +6,13 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { python-version: 3.9, os: ubuntu-latest }
-          - { python-version: 3.9, os: windows-latest }
-          - { python-version: 3.9, os: macos-latest }
-          - { python-version: 3.8, os: ubuntu-latest }
-          - { python-version: 3.7, os: ubuntu-latest }
-          - { python-version: 3.6, os: ubuntu-latest }
+          - { python-version: "3.10", os: ubuntu-latest }
+          - { python-version: "3.10", os: windows-latest }
+          - { python-version: "3.10", os: macos-latest }
+          - { python-version: "3.9", os: ubuntu-latest }
+          - { python-version: "3.8", os: ubuntu-latest }
+          - { python-version: "3.7", os: ubuntu-latest }
+          - { python-version: "3.6", os: ubuntu-latest }
     name: Python ${{ matrix.python-version }} (${{ matrix.os }})
     runs-on: ${{ matrix.os }}
     steps: