Refactored the tests to use pytest fixtures

[jacobsvante]

Also:

• Add linting and py35 to the test suite
• Running tox now tests MySQL, PostgreSQL and SQLite for all python versions
• Document how to pass in DSN string to tox for MySQL and PostgreSQL

[kvesteri]

Thanks for the PR! I fixed some issues, created another PR and merged that one :train:

[jacobsvante]

Great!