Eliminate the use of Python 2 in DAST

The last major release for Python 2 will be in April 2020 (see press release).

Remove references to Python 2 in the DAST scanner and confirm that it is only using python 3.

This will invalidate much of this issue: #14752 (closed)

Tasks:

• Work out which parts of zap-*.py in the zaproxy project are incompatible with Python 3
• Submit a merge request to the zaproxy authors making the code Python 2 and Python 3 compatible
• Wait for the merge request to be merged, and a new ZAP weekly image to be released
• Upgrade DAST code to use Python 3
• Choose a stable Python 3 version, and use it everywhere in our CI Pipeline
• Document and communicate which version of Python DAST engineers will need to use
• Add an entry to the changelog ready for release