ci: Travis: remove py38 from allowed failures; do not use "-dev"

.travis.yml

@@ -60,7 +60,7 @@ jobs:
     - env: TOXENV=py37-freeze
 
     - env: TOXENV=py38-xdist
-      python: '3.8-dev'
+      python: '3.8'
 
     - stage: baseline
       env: TOXENV=py36-xdist
@@ -94,11 +94,6 @@ jobs:
           tags: true
           repo: pytest-dev/pytest
 
-matrix:
-  allow_failures:
-    - python: '3.8-dev'
-      env: TOXENV=py38-xdist
-
 before_script:
   - |
     # Do not (re-)upload coverage with cron runs.