Get rid of non-NEP tests

Author: dstansby

.github/workflows/test_and_publish.yml

@@ -30,11 +30,6 @@ jobs:
         - linux: py311-test-mpl37
         - macos: py311-test-mpl37
         - windows: py311-test-mpl37
-        # Test oldest non-NEP 29 configurations
-        - linux: py39-test-mpl20
-          runs-on: ubuntu-20.04
-        - macos: py39-test-mpl20
-        - windows: py39-test-mpl20
         # Test newest configurations
         - linux: py312-test-mpl38
         - macos: py312-test-mpl38