Adds py37 to github actions workflow

Author: jasonb5

.github/workflows/srt.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
     env:
       CC: mpicc
       FC: mpifort