Upgrade to Python 3

What does this MR do?

• Installs Python3 on the DAST Docker image, and ensures that it is the system default.
• Upgrades pip to use Python3.
• Installed pip dependencies from the requirements.txt file. Previously, the file was unused and incorrect.

Why is this important?

• We currently run tests on Python 3.6, but use Python 2.7 on the image when running the analyzer. This creates unnecessary risk.
• Python 2.7 is due to be deprecated on the 1st of January 2020.

Considerations

• If this is to be considered a breaking change, then we should not release this until a new version of GitLab is released (13.0).
• I haven't considered it a breaking change, because functionality should not have changed from a user perspective.
• Note that the order of keys outputted in the gl-dast-report.json has changed. Our clients should not depend on the order, and neither do we, hence I haven't considered this a breaking change.
• @gitlab-org/secure/dynamic-analysis-be please note you will need to have Python3 installed on your local machine for development.