matrixed by linux version

Author: ericmjl

.azure-pipelines/linux.yml

@@ -6,13 +6,15 @@ variables:
 
 strategy:
   matrix:
-    py37:
+    linux-py37:
       python.version: "3.7"
-    py36:
+      vm.image: ubuntu-16.04
+    linux-py36:
       python.version: "3.6"
+      vm.image: ubuntu-16.04
 
 pool:
-  vmImage: ubuntu-16.04
+  vmImage: $(vm.image)
 
 steps:
 # Add conda to PATH.