python-single-r1.eclass: Restore PYTHON_SINGLE_TARGET for one impl

python-single-r1.eclass: Restore PYTHON_SINGLE_TARGET for one impl

Undo the hack that removes PYTHON_SINGLE_TARGET (in favor of plain
PYTHON_TARGETS) when the package supports only one target.  Instead,
output both kinds of flags consistently for all packages, and use IUSE
defaults to enable them when only one interpreter is supported.

This should give similar end results without creating unnecessary
differences between ebuilds with one and multiple implementations.
It will make future API changes easier, as well as simplify tooling
used to detect eclass type.

Signed-off-by: Michał Górny <mgorny@gentoo.org>