Bump macOS build image

Author: nis-spiir

.azure-pipelines/client.yml

@@ -71,7 +71,7 @@ jobs:
         python.version: '3.7'
       MacOS_Python27:
         os.name: 'MacOS'
-        os.vmImage: 'macOS-10.13'
+        os.vmImage: 'macOS-10.15'
         python.version: '2.7'
   pool:
     vmImage: '$(os.vmImage)'