run tests on ubuntu-20.04

and remove Python 3.4, since that's no longer available
on this test runner

.github/workflows/tests.yml

@@ -11,11 +11,11 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.4", "3.5", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "pypy3.9"]
+        python-version: ["3.5", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "pypy3.9"]
 
     steps:
     - uses: actions/checkout@v3