Add support for using ADDITIONAL_CA_CERT_BUNDLE with PIP_CERT

What does this MR do?

This MR adds support for using ADDITIONAL_CA_CERT_BUNDLE. This variable allows pip and pipenv to use pypi registries that have self-signed certificates. Both pip and pipenv already use PIP_CERT which allows users to supply a path to the certificate file on disk. This MR prefers PIP_CERT if both variables are set.

The integration tests for this change are:

• python-pip
• python-pipenv

Please note that all the tests are in one pipeline to make it easier to compare the jobs in .gitlab-ci.yml.

What are the relevant issue numbers?

gitlab-org/gitlab#214743 (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