Update Site

Follow this steps to update Python and pytest versions on the site:

Update pytest and/or python versions in .travis.yml:

python:
  - "2.7"
  - "3.6"
env:
  matrix:
  - PYTEST_VERSION=3.3.0 PLUGINCOMPAT_SITE=http://plugincompat.herokuapp.com

Update get_pytest_versions() and get_python_versions() in web.py to match the versions in .travis.yml.
Update test_versions in test_web.py.
Finally push master to GitHub and Heroku.

Update Requirements

Install pip-tools in a Python 3.7 virtual environment and execute:

$ pip install --upgrade -r requirements.in
$ pip-compile --upgrade

And commit everything.