BUG: special: fix sign of derivative when `x < 0` in `pbwa'
BUG: special: fix sign of derivative when `x < 0` in `pbwa'

Closes gh-7133.

endolith/scipy,mdhaber/scipy,anntzer/scipy,matthew-brett/scipy,jjhelmus/scipy,Stefan-Endres/scipy,matthew-brett/scipy,perimosocordiae/scipy,jakevdp/scipy,vigna/scipy,aarchiba/scipy,pizzathief/scipy,scipy/scipy,nmayorov/scipy,matthew-brett/scipy,arokem/scipy,arokem/scipy,vigna/scipy,nmayorov/scipy,jor-/scipy,jakevdp/scipy,aarchiba/scipy,lhilt/scipy,Stefan-Endres/scipy,anntzer/scipy,person142/scipy,aeklant/scipy,anntzer/scipy,aeklant/scipy,vigna/scipy,jjhelmus/scipy,perimosocordiae/scipy,aarchiba/scipy,pbrod/scipy,apbard/scipy,befelix/scipy,Stefan-Endres/scipy,pbrod/scipy,anntzer/scipy,arokem/scipy,zerothi/scipy,befelix/scipy,zerothi/scipy,jamestwebber/scipy,pbrod/scipy,apbard/scipy,ilayn/scipy,gfyoung/scipy,pizzathief/scipy,zerothi/scipy,jakevdp/scipy,e-q/scipy,person142/scipy,ilayn/scipy,rgommers/scipy,pizzathief/scipy,aeklant/scipy,Eric89GXL/scipy,grlee77/scipy,josephcslater/scipy,anntzer/scipy,lhilt/scipy,tylerjereddy/scipy,ilayn/scipy,rgommers/scipy,jor-/scipy,e-q/scipy,scipy/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,aeklant/scipy,jamestwebber/scipy,aeklant/scipy,gertingold/scipy,WarrenWeckesser/scipy,e-q/scipy,nmayorov/scipy,aarchiba/scipy,nmayorov/scipy,pizzathief/scipy,endolith/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,zerothi/scipy,ilayn/scipy,Eric89GXL/scipy,Eric89GXL/scipy,apbard/scipy,jjhelmus/scipy,tylerjereddy/scipy,lhilt/scipy,person142/scipy,gertingold/scipy,Stefan-Endres/scipy,lhilt/scipy,gfyoung/scipy,jakevdp/scipy,andyfaff/scipy,Eric89GXL/scipy,WarrenWeckesser/scipy,endolith/scipy,nmayorov/scipy,befelix/scipy,andyfaff/scipy,gertingold/scipy,rgommers/scipy,e-q/scipy,vigna/scipy,mdhaber/scipy,grlee77/scipy,matthew-brett/scipy,anntzer/scipy,person142/scipy,andyfaff/scipy,aarchiba/scipy,vigna/scipy,gfyoung/scipy,pizzathief/scipy,scipy/scipy,arokem/scipy,perimosocordiae/scipy,mdhaber/scipy,WarrenWeckesser/scipy,josephcslater/scipy,befelix/scipy,Eric89GXL/scipy,jamestwebber/scipy,befelix/scipy,ilayn/scipy,zerothi/scipy,gfyoung/scipy,Eric89GXL/scipy,jjhelmus/scipy,jjhelmus/scipy,arokem/scipy,rgommers/scipy,josephcslater/scipy,gertingold/scipy,matthew-brett/scipy,jor-/scipy,scipy/scipy,josephcslater/scipy,apbard/scipy,jakevdp/scipy,perimosocordiae/scipy,endolith/scipy,lhilt/scipy,mdhaber/scipy,pbrod/scipy,endolith/scipy,gertingold/scipy,andyfaff/scipy,mdhaber/scipy,ilayn/scipy,jor-/scipy,person142/scipy,scipy/scipy,zerothi/scipy,e-q/scipy,grlee77/scipy,pbrod/scipy,josephcslater/scipy,perimosocordiae/scipy,rgommers/scipy,grlee77/scipy,Stefan-Endres/scipy,jamestwebber/scipy,endolith/scipy,jamestwebber/scipy,jor-/scipy,scipy/scipy,apbard/scipy,tylerjereddy/scipy,tylerjereddy/scipy,grlee77/scipy,mdhaber/scipy,andyfaff/scipy,andyfaff/scipy,gfyoung/scipy,pbrod/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy