python-utils-r1.eclass: Reorder implementations to semi-ascending order

python-utils-r1.eclass: Reorder implementations to semi-ascending order

Reorder the Python implementations to ascending version order, with
CPython listed first and other implementations in descending preference.

The previous ordering has been used for two reasons:

1. There were packages which supported Python 3.x or PyPy partially but
their documentation builds or test functions required CPython 2.x.
The specific ordering caused python_export_best (the predecessor of
python_setup) to use CPython 2.x for those tasks. This is now replaced
by explicit implementation restrictions in python_setup.

2. PyPy setup runs were usually slower than CPython, and CPython 3.x
runs were often slower due to 2to3 calls. Combined with parallel build
runs, this ordering caused slower builds to start earlier and sometimes
resulted in more efficient use of threads. However, nowadays we no
longer do parallel builds.

Therefore, it seems reasonable to finally reorder the implementations
into a more intuitive order.