Add ability to specify requirements files for scanning

What does this MR do?

This MR adds a new env var that allows users to specify a list of requirements files not currently supported by gemnasium-python.

What are the relevant issue numbers?

gitlab-org/gitlab#8524 (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
  • [-] Auto-DevOps template
  • [-] Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• [-] Security reports checked/validated by reviewer

Test for this feature

See this pipeline and this branch for details.