Stop testing on -dev, they are broken on travis-ci

pip is missing on recent builds; and remove duplicate 3.7 testing if we
drop -dev suffix

.travis.yml

@@ -75,11 +75,7 b' matrix:'
       env: ARM64=True
       sudo: true
     - arch: amd64
-      python: "3.8-dev"
-      dist: xenial
-      sudo: true
-    - arch: amd64
-      python: "3.7-dev"
+      python: "3.8"
       dist: xenial
       sudo: true
     - arch: amd64