Upgrade to python 3

What does this MR do?

Updates DAST to use Python 3 (specifically, 3.6.9). This is a requirement as the EOL for Python 2 is April 2020.

This has been attempted before in DAST, with the unfortunate result that a broken version of DAST was released.

I'm confident this won't happen again:

• ZAProxy Python scripts have been updated to be Python 3 compatible.
• DAST has end-to-end tests that verify if it works before it is released.

You can verify that Python 3 is indeed used by checking the logs of the scans in the test/output directory of the artifacts of the end-to-end test pipelines.

This issue solves gitlab-org/gitlab#118748 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer