fix: remove Python 3.4, add 3.8, 3.9

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index ec8e9e5..5763d29 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,10 @@
 language: python
 python:
-- '3.4'
 - '3.5'
 - '3.6'
 - '3.7'
+- '3.8'
+- '3.9'
 install:
 - pip install -r requirements.txt
 - pip install coveralls