Update python-ci.yml

Author: ayodejidev

.github/workflows/python-ci.yml

@@ -13,10 +13,10 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: ['3.8', '3.9', '3.10']
+        python-version: ['3.6', '3.7', '3.8']
 
     steps:
       - name: Checkout