bump azure pipelines os version (#323)

* bump azure pipelines os version

* missed two jobs

Author: lboeman

azure-pipelines.yml

@@ -10,7 +10,7 @@ jobs:
 
 - job: 'Test'
   pool:
-    vmImage: 'Ubuntu-16.04'
+    vmImage: 'Ubuntu-latest'
   strategy:
     matrix:
       Python37:
@@ -76,7 +76,7 @@ jobs:
 - job: 'Publish'
   dependsOn: 'Test'
   pool:
-    vmImage: 'Ubuntu-16.04'
+    vmImage: 'Ubuntu-latest'
 
   steps:
   - task: UsePythonVersion@0
@@ -90,7 +90,7 @@ jobs:
 
 - job: 'TestDatabase'
   pool:
-    vmImage: 'Ubuntu-16.04'
+    vmImage: 'Ubuntu-latest'
 
   steps:
   - task: UsePythonVersion@0