Use Python 3.8 in CI tasks

Author: marco-c

.taskcluster.yml

@@ -60,7 +60,7 @@ tasks:
       deadline: {$fromNow: '1 hour'}
       payload:
         maxRunTime: 3600
-        image: python:3.7
+        image: python:3.8
         command:
           - sh
           - -lxce
@@ -81,7 +81,7 @@ tasks:
       deadline: {$fromNow: '1 hour'}
       payload:
         maxRunTime: 3600
-        image: python:3.7
+        image: python:3.8
         command:
           - sh
           - -lxce
@@ -103,7 +103,7 @@ tasks:
       deadline: {$fromNow: '1 hour'}
       payload:
         maxRunTime: 3600
-        image: python:3.7
+        image: python:3.8
         command:
           - sh
           - -lxce
@@ -124,7 +124,7 @@ tasks:
       deadline: {$fromNow: '1 hour'}
       payload:
         maxRunTime: 3600
-        image: python:3.7
+        image: python:3.8
         command:
           - sh
           - -lxce
@@ -144,7 +144,7 @@ tasks:
       deadline: {$fromNow: '1 hour'}
       payload:
         maxRunTime: 3600
-        image: python:3.7
+        image: python:3.8
         command:
           - sh
           - -lxce