MAINT: signal: Some PEP-8 clean up in _filter_design.py
MAINT: signal: Some PEP-8 clean up in _filter_design.py

ilayn/scipy,perimosocordiae/scipy,ilayn/scipy,andyfaff/scipy,andyfaff/scipy,perimosocordiae/scipy,rgommers/scipy,mdhaber/scipy,Stefan-Endres/scipy,ilayn/scipy,WarrenWeckesser/scipy,rgommers/scipy,scipy/scipy,anntzer/scipy,perimosocordiae/scipy,endolith/scipy,anntzer/scipy,Stefan-Endres/scipy,endolith/scipy,endolith/scipy,Stefan-Endres/scipy,Stefan-Endres/scipy,endolith/scipy,mdhaber/scipy,andyfaff/scipy,mdhaber/scipy,mdhaber/scipy,anntzer/scipy,ilayn/scipy,rgommers/scipy,anntzer/scipy,WarrenWeckesser/scipy,endolith/scipy,WarrenWeckesser/scipy,andyfaff/scipy,anntzer/scipy,andyfaff/scipy,Stefan-Endres/scipy,ilayn/scipy,scipy/scipy,perimosocordiae/scipy,perimosocordiae/scipy,mdhaber/scipy,anntzer/scipy,WarrenWeckesser/scipy,scipy/scipy,WarrenWeckesser/scipy,scipy/scipy,rgommers/scipy,andyfaff/scipy,ilayn/scipy,rgommers/scipy,perimosocordiae/scipy,mdhaber/scipy,endolith/scipy,Stefan-Endres/scipy,scipy/scipy,scipy/scipy,WarrenWeckesser/scipy