test: upgrade Linux Travis CI to Python 3.8

Ensure that we remain compatible with the newest version of Python.

PR-URL: #1923
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

.travis.yml

@@ -29,16 +29,16 @@ matrix:
         NODE_GYP_FORCE_PYTHON=/c/Python27/python.exe
       before_install: choco install python2
 
-    - name: "Node.js 6 & Python 3.7 on Linux"
-      python: 3.7
+    - name: "Node.js 6 & Python 3.8 on Linux"
+      python: 3.8
       env: NODE_GYP_FORCE_PYTHON=python3
       before_install: nvm install 6
-    - name: "Node.js 8 & Python 3.7 on Linux"
-      python: 3.7
+    - name: "Node.js 8 & Python 3.8 on Linux"
+      python: 3.8
       env: NODE_GYP_FORCE_PYTHON=python3
       before_install: nvm install 8
-    - name: "Node.js 10 & Python 3.7 on Linux"
-      python: 3.7
+    - name: "Node.js 10 & Python 3.8 on Linux"
+      python: 3.8
       env: NODE_GYP_FORCE_PYTHON=python3
       before_install: nvm install 10
 
@@ -54,6 +54,10 @@ matrix:
       python: 3.7
       env: NODE_GYP_FORCE_PYTHON=python3
       before_install: nvm install 12
+    - name: "Node.js 12 & Python 3.8 on Linux"
+      python: 3.8
+      env: NODE_GYP_FORCE_PYTHON=python3
+      before_install: nvm install 12
 
     - name: "Python 3.7 on macOS"
       os: osx