Most likely, OneGet won't replace pip/PyPI, any more than apt or yum does; but it may be worth having Python itself available that way. That might simply mean having someone package up Python and put it on an appropriate server, or maybe python.org could end up hosting a repo.
Python is already available in Chocolatey:
Given that OneGet intends to support Chocolatey as a repository, it might just work already. All python.org would have to guarantee is stable URLs for the Python msi files.
I'd like it if we guarantee stable URLs anyway. The 3.5 installer will have a single flag to switch between building a full bundle (~23MB) or a tiny downloader (<500KB), but the latter option is only viable if the URLs are stable. I can also include options in the downloader for 32/64-bit versions and debug symbols, which would really reduce the choices for a user (and yes - the entire installer is still automatable and I'll write up docs to go with it for sysadmin scenarios).
(I believe that OneGet does support dependencies, since one of the intended uses is setting up dev environments. If it gets backported, I'll see about setting up a Python build environment in there, unless someone else does it first.)