Delete Python 3.3 and add Python 3.8-dev on Travis

Currently Travis fails with:

    Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-3.3.tar.bz2
    $ curl -sSf -o python-3.3.tar.bz2 ${archive_url}
    curl: (22) The requested URL returned error: 404 Not Found
    Unable to download 3.3 archive. The archive may not exist. Please consider a different version.

Author: orivej

.travis.yml

@@ -1,10 +1,10 @@
 language: python
 python:
-  - "3.7-dev"
+  - "3.8-dev"
+  - "3.7"
   - "3.6"
   - "3.5"
   - "3.4"
-  - "3.3"
   - "2.7"
 install:
   - pip install nose