Why not use Jython? The only downside I can immediately think of is if your library uses CPython native extensions.
EDIT: If you can use Jython now but think you may have problems with a later version of the library, I suggest you try to isolate the library from your app (e.g. some sort of adapter interface). Go with the simplest thing that works for the moment, then consider JNI/CPython/etc if and when you ever need to. There's little to be gained by going the (painful) JNI route unless you really have to.
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