PEP 503
A repository MAY include a data-requires-python attribute on a file link. This exposes the Requires-Python metadata field, specified in PEP 345, for the corresponding release. Where this is present, installer tools SHOULD ignore the download when installing to a Python version that doesn’t satisfy the requirement. For example:
<a href="..." data-requires-python=">=3">...</a>
In the attribute value, < and > have to be HTML encoded as < and >, respectively.