GitHub Actions: test pypy3.9, remove pypy3.7

With pypy3.7 there were test failures related to lxml. I assume these started to
occur because lxml 5.1.0 is actually incompatible with pypy3.7 but does not declare
that version as incompatible.

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", "3.11", "3.12", "pypy-3.7", "pypy-3.8"]
+        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", "3.11", "3.12", "pypy-3.8", "pypy-3.9"]
 
     steps:
     - uses: actions/checkout@v4