[travis] Reduce to single Linux build

Switching main builds to github actions, but keeping
travis with a single linux build for the time being.

Author: mpsonntag

.travis.yml

@@ -4,26 +4,10 @@ dist: trusty
 language: python
 
 matrix:
-  allow_failures:
-    - os: linux
-      python: "3.9-dev"
-      dist: bionic
-
   include:
-    - os: linux
-      python: "3.5"
-    - os: linux
-      python: "3.6"
-      env: COVERALLS=1
-    - os: linux
-      python: "3.7"
-      dist: xenial
     - os: linux
       python: "3.8"
       dist: xenial
-    - os: linux
-      python: "3.9-dev"
-      dist: bionic
 
 install:
   - export PYVER=${TRAVIS_PYTHON_VERSION:0:1}