Hold over of old GPL licensing in setup.py

[hyandell]

With the project having relicensed from GPL-3.0 to MIT in 4a9b4e5

I think an update was missed to the setup.py metadata where

ydata-synthetic/setup.py

Line 32 in 251facc

'License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)',

still refers to GPL-3.0.