restrict MacOS tests in travis

Author: h-g-s

Diff for: .travis.yml

@@ -23,22 +23,7 @@ jobs:
     - name: "Python 3.8 on Linux"
       os: linux
       python: 3.8
-    - name: "Python 3.6 on macOS 10.13"
-      os: osx
-      osx_image: xcode9.4
-      env: PYTHON=36
-      language: shell # 'language: python' is an error on Travis CI macOS
-    - name: "Python 3.7 on macOS 10.13"
-      os: osx
-      osx_image: xcode9.4
-      env: PYTHON=37
-      language: shell # 'language: python' is an error on Travis CI macOS
-    - name: "Python 3.6 on macOS 10.14.6"
-      os: osx
-      osx_image: xcode11.4
-      env: PYTHON=36
-      language: shell # 'language: python' is an error on Travis CI macOS
-    - name: "Python 3.6 on macOS 10.14.6"
+   - name: "Python 3.7 on macOS 10.14.6"
       os: osx
       osx_image: xcode11.4
       env: PYTHON=37