Test python 3.6 7 and 8 for Xenial Linux

.travis.yml

@@ -1,16 +1,22 @@
 language: python
+linux_before_install: &linuxbefore
+  - sudo apt-get install -y texlive-latex-base pgf texlive-pictures texlive-latex-extra
+  - sudo apt-get install -y libblas-dev liblapack-dev libatlas-base-dev gfortran
 
 jobs:
   include:
-    - name: "Python on Xenial Linux"
+    - name:  "Python 3.6 on Xenial Linux"
       dist: xenial
-      python:
-        - "3.6"
-        - "3.7"
-        - "3.8"
-      before_install:
-        - sudo apt-get install -y texlive-latex-base pgf texlive-pictures texlive-latex-extra
-        - sudo apt-get install -y libblas-dev liblapack-dev libatlas-base-dev gfortran
+      before_install: *linuxbefore
+      python: 3.6
+    - name: "Python 3.7 on Xenial Linux"
+      dist: xenial
+      before_install: *linuxbefore
+      python: 3.7
+    - name: "Python 3.8 on Xenial Linux"
+      dist: xenial
+      before_install: *linuxbefore
+      python: 3.8
     - name: "Python 3.7.4 on macOS"
       os: osx
       osx_image: xcode11.2  # Python 3.7.4 running on macOS 10.14.4