Run CI for Python 3.6 on Ubuntu 20.04

Author: sisp

.github/workflows/cicd.yml

@@ -4,13 +4,12 @@ on: [push, pull_request]
 
 jobs:
   ci:
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ matrix.python-version == '3.6' && 'ubuntu-20.04' || 'ubuntu-latest' }}
     strategy:
       fail-fast: false
       max-parallel: 4
       matrix:
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
-        os: [ubuntu-latest]
 
     steps:
       - uses: actions/checkout@v1