add 3.7 tests

Author: guiwitz

.github/workflows/test_build.yml

@@ -17,7 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         platform: [ ubuntu-latest, windows-latest, macos-latest ]
-        python-version: [ 3.8, 3.9 ]
+        python-version: [ 3.7, 3.8, 3.9 ]
 
     steps:
       - uses: actions/checkout@v2