It's true that PySide's future on Qt 5 is uncertain, but that's not the case for PyQt. So technically, it should be possible to do this.
Then there's the point of politics: Canonical said they will release the source code for Ubuntu Phone in due time, so it's very likely that someone hacks support for Python in it after that. If Canonical itself will support that is not clear yet, as far as I'm aware they didn't make a statement on that yet. Nor is it clear for any OEMs/operators (because there aren't any (known) yet).
UPDATE: Canonical released the source (as expected), and some operators are known now. It seems like Canonical's not planning to add Python support for the SDK themselves (at least for now), but that they're open to contributions.
There is no official support for pyqt. Last time I looked at pyqt, it
didn't even support qt5, which the phone is based on. I don't think
you'll have much luck with it.