CI unit tests: one macOS build is enough

Change-Id: I8b1d1287612f3994eb55e6c2fc48987361ee6c22
diff --git a/.github/workflows/pythonpackage.yml b/.github/workflows/pythonpackage.yml
index 41ffa15..7c09d55 100644
--- a/.github/workflows/pythonpackage.yml
+++ b/.github/workflows/pythonpackage.yml
@@ -15,6 +15,8 @@
         exclude:
           - os: macOS-latest
             python-version: 3.7
+          - os: macOS-latest
+            r-version: 3.6
           - os: windows-latest
             python-version: 3.7
           - os: windows-latest