Run CI on windows too

Wonder if this works.

Author: Hugo Osvaldo Barrera

.github/workflows/tests.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-20.04, macOS-10.15 ]
+        os: [ ubuntu-20.04, macOS-10.15, windows-2019 ]
         python: [ '3.6', '3.7', '3.8' ]
         variant: [ "py", "py-images" ]
     name: python${{ matrix.python }} on ${{ matrix.os }} ${{ matrix.variant }}