gh-actions: upgrade to generic R release version

Change-Id: Icb59c32a20710c98d90e8af3fe6bf6847eafe7fe
diff --git a/.github/workflows/pythonpackage.yml b/.github/workflows/pythonpackage.yml
index 2983edf..6eba34c 100644
--- a/.github/workflows/pythonpackage.yml
+++ b/.github/workflows/pythonpackage.yml
@@ -7,22 +7,22 @@
 
     runs-on: ${{ matrix.os }}
     strategy:
-      max-parallel: 4
+      fail-fast: false
       matrix:
         python-version: [3.7, 3.8]
-        r-version: [3.6, 4.0]
+        r-version: [3.6, release]
         os: [windows-latest, macOS-latest, ubuntu-20.04]
         exclude:
           - os: macOS-latest
             python-version: 3.7
           - os: macOS-latest
-            r-version: 4.0
+            r-version: 3.6
           - os: windows-latest
             python-version: 3.7
           - os: windows-latest
             r-version: 3.6
           - os: ubuntu-20.04
-            r-version: 4.0
+            r-version: 3.6
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}