CI: add Windows Python 3.9 to CI
CI: add Windows Python 3.9 to CI

* we do not currently have SciPy tested in CI for
Windows with Python 3.9; this may also be useful
to have in place before releasing 3.9 wheels on
the `1.5.x` branch

Eric89GXL/scipy,grlee77/scipy,anntzer/scipy,matthew-brett/scipy,anntzer/scipy,tylerjereddy/scipy,matthew-brett/scipy,perimosocordiae/scipy,e-q/scipy,endolith/scipy,grlee77/scipy,perimosocordiae/scipy,anntzer/scipy,e-q/scipy,mdhaber/scipy,grlee77/scipy,endolith/scipy,anntzer/scipy,andyfaff/scipy,endolith/scipy,vigna/scipy,zerothi/scipy,rgommers/scipy,matthew-brett/scipy,zerothi/scipy,Eric89GXL/scipy,ilayn/scipy,andyfaff/scipy,andyfaff/scipy,scipy/scipy,rgommers/scipy,Eric89GXL/scipy,ilayn/scipy,WarrenWeckesser/scipy,zerothi/scipy,Eric89GXL/scipy,vigna/scipy,Stefan-Endres/scipy,scipy/scipy,rgommers/scipy,tylerjereddy/scipy,ilayn/scipy,Stefan-Endres/scipy,vigna/scipy,scipy/scipy,andyfaff/scipy,zerothi/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,anntzer/scipy,grlee77/scipy,ilayn/scipy,nmayorov/scipy,zerothi/scipy,e-q/scipy,Stefan-Endres/scipy,tylerjereddy/scipy,WarrenWeckesser/scipy,mdhaber/scipy,tylerjereddy/scipy,ilayn/scipy,scipy/scipy,perimosocordiae/scipy,perimosocordiae/scipy,perimosocordiae/scipy,Eric89GXL/scipy,mdhaber/scipy,endolith/scipy,andyfaff/scipy,WarrenWeckesser/scipy,mdhaber/scipy,matthew-brett/scipy,e-q/scipy,scipy/scipy,ilayn/scipy,matthew-brett/scipy,Eric89GXL/scipy,andyfaff/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,endolith/scipy,mdhaber/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,Stefan-Endres/scipy,zerothi/scipy,vigna/scipy,nmayorov/scipy,nmayorov/scipy,nmayorov/scipy,rgommers/scipy,vigna/scipy,anntzer/scipy,endolith/scipy,rgommers/scipy,nmayorov/scipy,grlee77/scipy,scipy/scipy,e-q/scipy,mdhaber/scipy