CI: fix pipeline using latest ubuntu (#13)

ci/main.yml

@@ -19,9 +19,9 @@ stages:
         cancelTimeoutInMinutes: 5
         strategy:
           matrix:
-            linux_python36:
+            linux_python38:
-              imageName: "ubuntu-16.04"
+              imageName: "ubuntu-latest"
-              python.version: "3.6"
+              python.version: "3.8"
               WHEELS_PATH: "/dist/*.whl"
             # macos1015_python36:
             #   imageName: 'macOS-10.15'