increase python version in github actions

Author: ortega2247

.github/workflows/build.yml

@@ -6,10 +6,10 @@ jobs:
 
     runs-on: ${{ matrix.os }}
     strategy:
-      max-parallel: 4
+      max-parallel: 6
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6, 3.7]
+        python-version: [3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@master