Updated appveyor cfg

Author: Panos

.appveyor.yml

@@ -19,6 +19,7 @@ environment:
     # Python versions to build wheels for
     PYTHONVERS: C:\Python36-x64 C:\Python37-x64 C:\Python38-x64 C:\Python39-x64
     PYTHON_ARCH: "64"
+    MSVC: "Visual Studio 16 Win64"
 
 install:
   # If there is a newer build queued for the same PR, cancel this one.