Use pytest, test dependency versions

[davidism]

Flask converted, we should too. They also test at the minimal, current, and development versions of their dependencies. I don't think we need to go development, but we definitely need to test at SQLAlchemy==0.8.

• convert to pytest
• split tests into multiple files
• add tests at minimal dependencies to tox
• adjust travis to use new test targets

[davidism]

added tox/travis envs in #388

[thrawny]

Are looking for help with converting to pytest? I would like to help out.

[davidism]

@thrawny yes, that would be a big help! Some things I'd like to do:

• split the tests into different files instead of one big one
• use flat functions instead of classes
• use fixtures where appropriate

Ultimately, I'd like to rewrite the whole test suite with consistency and coverage in mind, but you don't have to go that far. You can see Flask-WTF's test suite as an example, I rewrote that a while ago: https://github.com/lepture/flask-wtf/tree/master/tests

[thrawny]

@davidism Great! Thanks for the input. Will start working on it then.