ci: Update python matrix (GoogleCloudPlatform#225)

Signed-off-by: GitHub <[email protected]>
Co-authored-by: chizhg <[email protected]>

Author: chizhg

.github/workflows/conformance.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-18.04
     strategy:
       matrix:
-        python: ['3.8', '3.9', '3.10', '3.11']
+        python: ['3.7', '3.8', '3.9', '3.10', '3.11']
     steps:
     - name: Harden Runner
       uses: step-security/harden-runner@18bf8ad2ca49c14cbb28b91346d626ccfb00c518 # v2.1.0