CI: Lower Python version to 3.5

Ensure that we test with the oldest version of Python 3 supported for
building, to avoid accidentally breaking the build.

(cherry picked from commit 00c7816)

Author: aperezdc

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v1
         with:
-          python-version: 3.6
+          python-version: 3.5
       - name: Python Package Cache
         uses: actions/cache@v1
         with: