Add Python 2.7 to testing matrix

Author: pganssle

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [2.7, 3.6, 3.7, 3.8]
         os: ["ubuntu-latest", "windows-latest", "macos-latest"]
     env:
       TOXENV: py