GitHub Action: Add Python 3.10 and remove 3.6

https://devguide.python.org/#status-of-python-branches vs.
https://devguide.python.org/devcycle/#end-of-life-branches

Author: cclauss

.github/workflows/main.yml

@@ -13,15 +13,15 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
         os: [macOS-latest, ubuntu-latest, windows-latest]
 
     steps:
       - name: Checkout
         uses: actions/checkout@v2
 
       - name: Set Up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v3
         with:
           python-version: ${{ matrix.python-version }}