private pypi repo not a valid HTML document

When using a private pypi repo to host a python package, pip complains with the following deprecation warning when installing a package from such a repo:

DEPRECATION: The HTML index page being used (https://gitlab.***.com/api/v4/projects/***/packages/pypi/simple/***/) is not a proper HTML 5 document. This is in violation of PEP 503 which requires these pages to be well-formed HTML 5 documents. Please reach out to the owners of this index page, and ask them to update this index page to a valid HTML 5 document. pip 22.2 will enforce this behaviour change. Discussion can be found at https://github.com/pypa/pip/issues/10825

(project names redacted, this is a self-hosted gitlab, but it's up to date with the June release, and I haven't found any related closed issues since then)

The mentioned pip 22.2 is already released, so some urgency may be required.