I have a project that I have licensed under the GPL3. In it, I have a section of code which implements a bessel filter, which I ported from the python scipy implementation. I'm unclear on how scipy code is licensed - is it OK to simply include that code in my GPL project, or do I have to add something additional since it was derived from a python library?
Scipy proper has a clear BSD license so there is not un-clear.
When porting code, I prefer to keep the original license of the code in all cases (even if the license may not require it).
I would typically port the files in files that match more or less the ported files structure and keep these clearly under their original license. To avoid confusion and meet the BSD requirements I would also copy the Scipy license in a header comment in these files together with some notes explaining that this is port, available under the original ported code license and eventually describing some of the changes. Not all of this is required, but this is to me the right thing to do.
As Philippe notes, scipy is licensed under the 3-clause BSD license. The FSF believes this to be a GPL-compatible license, which means you should have no problems incorporating it into your project and licensing the result under the GPLv3.