The Qt Company has released official and pip-installable PySide2 wheel snapshots!
Simo posted the details today on the PySide mailing list, but in a nutshell:
pip install --index-url=http://download.qt.io/snapshots/ci/pyside/5.9/latest/ pyside2 --trusted-host download.qt.io
Update 2018-04-05: official docs are now also online at https://doc-snapshots.qt.io/qtforpython/
Update 2018-07-17: PySide2 can now be installed from pypi.org:
pip install PySide2!