Add dependencies to tox.ini file

Right now if someone outside of Jenkins wants to run the tests our dev guide has them run the tests in their current Python env ( [https://beam.apache.org/contribute/contribution-guide/](https://beam.apache.org/contribute/contribution-guide/) ). However as we move to supporting Python 3 & 2 it would be good if they could use tox in the same way we do for linting. On Jenkins the requirements seems to already be installed, but for other folks this may not be the case so listing the requirements in the deps= would be good.

Imported from Jira [BEAM-3984](https://issues.apache.org/jira/browse/BEAM-3984). Original Jira may contain additional context.
Reported by: holden.