Use Ubuntu 20.04 for py36

Author: ConorMacBride

Diff for: .github/workflows/test_and_publish.yml

@@ -25,8 +25,11 @@ jobs:
         - windows: py310-test-mpl36
         # Test all configurations on Linux
         - linux: py36-test-mpl20
+          runs-on: ubuntu-20.04
         - linux: py36-test-mpl21
+          runs-on: ubuntu-20.04
         - linux: py36-test-mpl22
+          runs-on: ubuntu-20.04
         - linux: py37-test-mpl30
         - linux: py37-test-mpl31
         - linux: py37-test-mpl32