Test new Travis CPUs on a single Python version each

.travis.yml

@@ -6,43 +6,23 @@ notifications:
   irc: "chat.freenode.net#pil"
 
 # Run fast lint first to get fast feedback.
-# Run slow PyPy next, to give it a headstart and reduce waiting time.
-# Run latest 3.x next, to get quick compatibility results.
-# Then run the remainder.
+# Run slower CPUs next, to give them a headstart and reduce waiting time.
+# Then run the remainder, with fastest Docker jobs last.
 
 matrix:
   fast_finish: true
   include:
-    - python: "3.8"
-      arch: arm64
-    - python: "3.7"
-      arch: arm64
     - python: "3.6"
-      arch: arm64
-    - python: "3.5"
-      arch: arm64
+      name: "Lint"
+      env: LINT="true"
 
-    - python: "3.8"
-      arch: ppc64le
-    - python: "3.7"
-      arch: ppc64le
     - python: "3.6"
-      arch: ppc64le
-    - python: "3.5"
-      arch: ppc64le
-
-    - python: "3.8"
-      arch: s390x
+      arch: arm64
     - python: "3.7"
-      arch: s390x
-    - python: "3.6"
-      arch: s390x
+      arch: ppc64le
     - python: "3.5"
       arch: s390x
 
-    - python: "3.6"
-      name: "Lint"
-      env: LINT="true"
     - python: "pypy3"
       name: "PyPy3 Xenial"
     - python: "3.8"