7.0b: Python 3 and Thumbor 7 pre-release

Beta version to test Python 3 and Thumbor 7

What's Changed

• Refactor to use aiobotocore, python3 and tornado 7 by @amanagr in #147
• Use pytest instead of deprecated nose by @peterrus in #153

New Contributors

• @amanagr made their first contribution in #147
• @peterrus made their first contribution in #153

Full Changelog: 6.2.15...7.0b