Add signbit to math.pxd.
Add signbit to math.pxd.

Also reorder a few imports to alphabetical order.

dahebolangkuan/cython,acrispin/cython,mrGeen/cython,andreasvc/cython,slonik-az/cython,cython/cython,madjar/cython,cython/cython,dahebolangkuan/cython,roxyboy/cython,fabianrost84/cython,scoder/cython,hickford/cython,encukou/cython,fabianrost84/cython,acrispin/cython,achernet/cython,encukou/cython,ChristopherHogan/cython,hhsprings/cython,andreasvc/cython,mcanthony/cython,c-blake/cython,JelleZijlstra/cython,mcanthony/cython,fabianrost84/cython,hickford/cython,dahebolangkuan/cython,encukou/cython,mcanthony/cython,encukou/cython,c-blake/cython,c-blake/cython,slonik-az/cython,andreasvc/cython,fperez/cython,slonik-az/cython,achernet/cython,mrGeen/cython,mcanthony/cython,hickford/cython,marscher/cython,achernet/cython,mrGeen/cython,marscher/cython,roxyboy/cython,da-woods/cython,hickford/cython,da-woods/cython,JelleZijlstra/cython,marscher/cython,dahebolangkuan/cython,roxyboy/cython,madjar/cython,mcanthony/cython,fperez/cython,fperez/cython,encukou/cython,scoder/cython,c-blake/cython,hhsprings/cython,da-woods/cython,roxyboy/cython,acrispin/cython,fabianrost84/cython,marscher/cython,fperez/cython,madjar/cython,fperez/cython,madjar/cython,hhsprings/cython,andreasvc/cython,cython/cython,cython/cython,fabianrost84/cython,slonik-az/cython,dahebolangkuan/cython,da-woods/cython,achernet/cython,mrGeen/cython,marscher/cython,c-blake/cython,mrGeen/cython,scoder/cython,hhsprings/cython,JelleZijlstra/cython,acrispin/cython,madjar/cython,JelleZijlstra/cython,scoder/cython,roxyboy/cython,acrispin/cython,slonik-az/cython,hickford/cython,achernet/cython,JelleZijlstra/cython,andreasvc/cython,hhsprings/cython,ChristopherHogan/cython,ChristopherHogan/cython