Use python 3.8 for aarch64, ppc64le & s390x tests

Author: mayeut

.travis.yml

@@ -8,15 +8,15 @@ jobs:
   - python: "3.7"
   - python: "3.8"
   - python: "3.9"
-  - python: "3.7"
+  - python: "3.8"
     arch: arm64-graviton2
     virt: vm
     group: edge
-  - python: "3.7"
+  - python: "3.8"
     arch: ppc64le
-  - python: "3.7"
+  - python: "3.8"
     arch: s390x
-  - python: "3.7"
+  - python: "3.8"
     env: LINTER=1
 
 services: