gh-actions: use generic ubuntu-latest

Change-Id: Ida22f57609b2792e79186b8f38a4e4298064ce92
diff --git a/.github/workflows/pythonpackage.yml b/.github/workflows/pythonpackage.yml
index 0dc1ba5..b3db0d4 100644
--- a/.github/workflows/pythonpackage.yml
+++ b/.github/workflows/pythonpackage.yml
@@ -12,7 +12,7 @@
       matrix:
         python-version: [3.7, 3.8]
         r-version: [3.6, release]
-        os: [windows-latest, macOS-latest, ubuntu-20.04]
+        os: [windows-latest, macOS-latest, ubuntu-latest]
         exclude:
           - os: macOS-latest
             python-version: 3.7
@@ -22,8 +22,9 @@
             python-version: 3.7
           - os: windows-latest
             r-version: 3.6
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
             r-version: 3.6
+            python-version: 3.8
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}