tests: Add ability to test different expected outputs based on Python version. Similar to the previous change, there was also a change in what gets escaped by `urllib.parse.quote` (as in: the tilde sign ('~') stopped being escaped). So now we need to be able to test different outputs in the tests, yay again.

sudo: false
language: python
python:
  - "3.4"
  - "3.5"
  - "3.6"
install:
  - "python setup.py version -f 2.0"
  - "python setup.py sdist --formats=zip"
  - "pip install dist/PieCrust-2.0.zip"
script:
  - "py.test"