ci: Bump on AppVeyor to MSVC 2019

According to the SciPy toolchain roadmap [1], we should be supporting at
minimum MSVC 2019. The AppVeyor image has been held back to MSVC 2017
(probably just forgotten since it didn't complain), which is starting to
cause issues for more modern code.

[1] https://docs.scipy.org/doc/scipy/dev/toolchain.html

Author: QuLogic

.appveyor.yml

@@ -17,7 +17,7 @@ skip_commits:
 
 clone_depth: 50
 
-image: Visual Studio 2017
+image: Visual Studio 2019
 
 environment: