Comply with PEP 518. (#7309)
Comply with PEP 518. (#7309)

BLD: add support for PEP 518.

* PEP 518: fixup

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