constraints.txt was originally used to provide version-constrained pip project specification to the toolchain-builder/paranoid devs. Without the evergreen verify_pip task, it's best to remove this feature all together since it will likely not be refreshed often if at all. The steps involved in this:

1. Rename etc/pip/toolchain-requirements.txt to etc/pip/complete-requirements.txt and include etc/pip/components/platform.req at the top. Use this file in etc/evergreen.yml.
2. Switch toolchain-builder to use etc/pip/complete-requirements.txt instead of etc/pip/constraints.txt
3. Remove etc/pip/constraints.txt and generate-pip-requirements.sh. Update etc/pip/README.md.