ENH: Update Python versions in CI

Drops support for Python 3.6 and adds support for Python 3.10 according
to ITK dependency.

Author: tbirdso

.github/workflows/build-test.yml

@@ -10,12 +10,12 @@ jobs:
       max-parallel: 4
       matrix:
         os: [ubuntu-latest, macos-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ['3.7', '3.8', '3.9', '3.10']
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies