Bug#834232: cysignals license

[Jeroen Demeyer <jdemeyer@cage.ugent.be>]

In case Debian cares about the difference between GPL and LGPL: the 
license of cysignals is *LGPL* version 3 or later. In full detail:

cysignals is free software: you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published
by the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

See also https://github.com/sagemath/cysignals/blob/master/LICENSE