app-pda/libplist: Switch to python-single-r1, disable py3

app-pda/libplist: Switch to python-single-r1, disable py3

Replace the semi-broken Python support code in ebuild with
python-single-r1. The original code used python-r1 and rebuilt the whole
package (incl. C libraries) for each implementation, even when Python
support was explicitly disabled.

Furthermore, the used autoconf macros break with Python 3.2+. Therefore,
the Python 3 support has been disabled entirely until the relevant code
is fixed.

Bug: https://bugs.gentoo.org/show_bug.cgi?id=567370