tests: update travis config

fix build: pytest needs to be updated
remove 3.3, it is EOL
add 3.7

Author: sebix

.travis.yml

@@ -1,23 +1,24 @@
+dist: xenial   # required for Python >= 3.7
 language: python
 env:
   - REQUIREMENTS=true
   - REQUIREMENTS=false
 python:
   - "2.7"
-  - "3.3"
   - "3.4"
   - "3.5"
   - "3.6"
-  - "pypy-5.4"
+  - "3.7"
+  # PyPy versions
+  - "pypy3.5"
 # command to install dependencies
 install:
   - if [[ $REQUIREMENTS == true ]] ; then pip install -r requirements.txt ; fi
-  - pip install coveralls pytest pytest-cov coverage codecov
+  - pip install -U coveralls pytest pytest-cov coverage codecov
   - pip install -e .
   - if [[ ! $TRAVIS_PYTHON_VERSION == 'pypy-5.4' ]] ; then pip install regex; fi
 # command to run tests
 script: py.test
-sudo: false
 after_success:
   - coveralls
   - codecov