Add Python 3.8 to the CI matrix (#58)

This also required bumping the flake8 version.

Author: person142

.travis.yml

@@ -1,10 +1,12 @@
 language: python
 matrix:
   include:
+    - python: 3.8
+      dist: xenial
     - python: 3.7
       dist: xenial
-      sudo: true
     - python: 3.6
+      dist: xenial
 
 notifications:
   email: false

test-requirements.txt

@@ -1,5 +1,5 @@
 black==18.9b0
-flake8==3.6.0
+flake8==3.7.9
 flake8-pyi==18.3.1
 pytest==4.0.0
 mypy==0.770