ci: Fix appveyor build specifying correct AppVeyor image
ci: Fix appveyor build specifying correct AppVeyor image

Adapted from  scikit-build/cmake-python-distributions@9bc10f963

Since scikit-build 0.10.0 more carefully tests availability of Visual Studio
generator, this commit fixes the issue using Visual Studio 2017 image
for python 3.6 and python 3.7

scikit-build/ninja-python-distributions