travis: disable some redundant tests to speed up pipeline (python>=3.5 in github workflow)

.travis.yml

@@ -1,14 +1,10 @@
 language: python
 
+# python>=3.5 in .github/workflows/python.yml
 python:
 - 3.4
 - 3.5
 - 3.6
-- 3.6-dev
-- 3.7
-- 3.7-dev
-- 3.8
-- 3.8-dev
 
 # required for python >= 3.7
 dist: xenial