I think this is the safe/conservative method of implementing a
required-use check in the -r1 eclasses.

Just like with PYTHON_DEPS, we leave it to the ebuild author to
utilize the value by adding it to REQUIRED_USE. I will certainly
assist in the mass-update that will be necessary to implement this in
existing ebuilds. This does not need to be done immediately.

The PYTHON_REQUIRED_USE name may cause a bit of confusion with the
PYTHON_REQ_USE variable, but I think the latter was a poor naming
choice to begin with.