temporarily disable CI tests on macosx (#470)

It seems that the package bou.ke/monkey is no longer supported with the
lastest macosx (it fails with a Permission denied error).

Author: cmaglie

.github/workflows/test.yaml

@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        operating-system: [ubuntu-18.04, windows-2019, macOS-10.14]
+        operating-system: [ubuntu-18.04, windows-2019]
 
     runs-on: ${{ matrix.operating-system }}