text: Title: Test factory_boy against Python 3.7
  Body: #### The problem
  ``factory_boy`` should be tested against python 3.7.
  
  #### Extra notes
  There might be some issues with configuring Travis-CI with python 3.7.
  
  #### Checklist:
  
  - [x] Add the target to ``tox.ini``
  - [ ] Add that target to ``.travis.yml``
  - [ ] Update the trove classifiers in ``setup.py``
  - [ ] Update the list of supported version in the docs