Eliminate unnecessary Travis CI jobs (#567)

We already have full tests (Python 3.6-3.8 on Linux, macOS and Windows)
running on GitHub Actions. No need to waste CI resources on Travis.

Author: seisman

.travis.yml

@@ -33,14 +33,6 @@ env:
 jobs:
     # Build under the following configurations
     include:
-        - name: "Linux - Python 3.8"
-          os: linux
-          env:
-              - PYTHON=3.8
-        - name: "Linux - Python 3.7"
-          os: linux
-          env:
-              - PYTHON=3.7
         - name: "Linux - Python 3.6 (deploy)"
           os: linux
           env:
@@ -67,11 +59,6 @@ script:
     # Build the documentation
     - make -C doc all
 
-# Things to do if the build is successful
-after_success:
-    # Upload coverage information
-    - codecov -e PYTHON
-
 deploy:
     # Make a release on PyPI
     - provider: script