TST Test multiple versions of Python

Python 3.[678] are currently supported.

Following examples at
https://help.github.com/en/actions/language-and-framework-guides/using-python-with-github-actions#specifying-a-python-version

.github/workflows/gmgc_mapper_test.yml

@@ -4,15 +4,19 @@ on: [push, pull_request]
 
 jobs:
   build:
-
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: [3.6, 3.7, 3.8]
+
 
     steps:
-    - uses: actions/checkout@v1
-    - name: Set up Python 3.7
-      uses: actions/setup-python@v1
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
       with:
-        python-version: 3.7
+        python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
         pip install numpy