Drop support for Ubuntu 20

P:er: actions/runner-images#11101

Author: jacobschaer

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest, ubuntu-20.04]
+        os: [ubuntu-latest, windows-latest]
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
         exclude:
           - os: ubuntu-latest