BLD: Fix Travis CI Python 3.7 on macOS, build on Python 3.8 on macOS and Linux

.travis.yml

@@ -6,26 +6,38 @@ notifications:
 
 matrix:
     include:
-        - name: "Python 2.7.15 on Xenial Linux"
-          python: 2.7           # this works for Linux but is ignored on macOS or Windows
+        # "python" version works for Linux, but is an error on macOS
+        - name: "Python 2.7 on Xenial Linux"
+          python: 2.7
           dist: xenial
-        - name: "Python 3.6.1 on Xenial Linux"
-          python: 3.6           # this works for Linux but is ignored on macOS or Windows
+        - name: "Python 3.6 on Xenial Linux"
+          python: 3.6
           dist: xenial
           env: DEPLOY_ENC_LABEL=e64cfe3b4e81
-        - name: "Python 3.7.1 on Xenial Linux"
-          python: 3.7           # this works for Linux but is ignored on macOS or Windows
+        - name: "Python 3.7 on Xenial Linux"
+          python: 3.7
           dist: xenial
-        - language: generic
+        - name: "Python 3.8 on Xenial Linux"
+          python: 3.8
+          dist: xenial
+        - name: "Python 2.7 on macOS"
+          language: generic
           os: osx
           env: TRAVIS_PYTHON_VERSION=2.7
-        - language: generic
+        - name: "Python 3.6 on macOS"
+          language: generic
           os: osx
           env: TRAVIS_PYTHON_VERSION=3.6
-        - name: "Python 3.7.2 on macOS"
+        - name: "Python 3.7 on macOS"
+          os: osx
+          osx_image: xcode10.1
+          language: generic
+          env: TRAVIS_PYTHON_VERSION=3.7
+        - name: "Python 3.8 on macOS"
           os: osx
-          osx_image: xcode10.2  # Python 3.7.2 running on macOS 10.14.3
-          language: generic       # 'language: python' is an error on Travis CI macOS
+          osx_image: xcode10.1
+          language: generic
+          env: TRAVIS_PYTHON_VERSION=3.8
 
 # Setup anaconda
 before_install: