Add Python 3.7 and 3.8 (actual stable) to Travis (#245)

* Add Python 3.7 and 3.8 (actual stable) to Travis

* Test if travis works with this

* Change win python version
Change expression to py

* Change back to linux only

.travis.yml

 language: python
 python:
   - "3.6"
+  - "3.7"
+  - "3.8"
   - "pypy3"
 # command to install dependencies
 install: