travis: Use Python 3.8 globally

.travis.yml

@@ -1,7 +1,7 @@
 dist: xenial
 language: python
 group: edge
-python: 3.7
+python: 3.8
 os: linux
 
 matrix:
@@ -15,28 +15,28 @@ matrix:
     - env: DOCKER=archlinux-webengine QUTE_BDD_WEBENGINE=true
       services: docker
 
-    ### PyQt 5.7.1 (Python 3.6)
-    - python: 3.6
-      env: TESTENV=py36-pyqt57
-
-    ### PyQt 5.9
-    - env: TESTENV=py37-pyqt59
-    ### PyQt 5.9 (Python 3.5)
+    ### PyQt 5.7.1 (Python 3.5)
     - python: 3.5
-      env: TESTENV=py35-pyqt59
+      env: TESTENV=py35-pyqt57
 
-    ### PyQt 5.10
-    - env: TESTENV=py37-pyqt510
+    ### PyQt 5.9 (Python 3.6)
+    - python: 3.6
+      env: TESTENV=py36-pyqt59
+
+    ### PyQt 5.10 (Python 3.6)
+    - python: 3.6
+      env: TESTENV=py36-pyqt510
       addons:
         apt:
           packages:
           - xfonts-base
 
-    ### PyQt 5.11
-    - env: TESTENV=py37-pyqt511
+    ### PyQt 5.11 (Python 3.7)
+    - python: 3.7
+      env: TESTENV=py37-pyqt511
 
-    ### PyQt 5.12
-    - env: TESTENV=py37-pyqt512
+    ### PyQt 5.12 (Python 3.8)
+    - env: TESTENV=py38-pyqt512
       # http://code.qt.io/cgit/qt/qtbase.git/commit/?id=c3a963da1f9e7b1d37e63eedded61da4fbdaaf9a
       addons:
         apt:
@@ -45,7 +45,6 @@ matrix:
 
     ### PyQt 5.13 (Python 3.8, with coverage)
     - env: TESTENV=py38-pyqt513-cov
-      python: 3.8-dev
       # http://code.qt.io/cgit/qt/qtbase.git/commit/?id=c3a963da1f9e7b1d37e63eedded61da4fbdaaf9a
       addons:
         apt:
@@ -57,6 +56,7 @@ matrix:
       env: TESTENV=py37-pyqt513 OSX=sierra
       osx_image: xcode9.2
       language: generic
+      python: 3.7
     ### macOS yosemite
     # https://github.com/qutebrowser/qutebrowser/issues/2013
     # - os: osx