The PEP 12 template for new PEPs clearly (and strongly) encourages PEP authors to dedicate the PEP contents to the public domain (for various reasons this expresses the intent well but is not legally sufficient, and probably PEPs should use CC-0, but that’s another topic). Alternatively, PEP authors can choose the Open Publication License.
However, sending a PR to the PEP repo on GitHub requires that the contributor sign the PSF Contributor Agreement, which has multiple effects. One is to indicate the license under which the contributions will be made (defaulting to Apache License Version 2), but that is not the case for PEP submissions as they would not be accepted under that license. It also grants the PSF permission to relicense the contributions under ‘any other open source license’ as decided by the PSF Board, but that is not relevant for PEP submissions that have been dedicated to the public domain.
I’m not a lawyer, although I do have quite a lot of knowledge in this area, and I handle these activities for my employer, but it seems to me that the repository for PEPs should be excluded from the PSF Contributor Agreement requirement, because the PEPs have their own licensing structure and requirements (and don’t use ‘software’ licenses anyway, since they are not software).