Avoid running tests under PyPy on GitHub Actions

Until we can identify what makes the connection to Solr unstable in the GitHub Actions environment, this is less important than not normalizing CI failures.

.github/workflows/tox.yml

@@ -19,7 +19,7 @@ jobs:
       fail-fast: false
       max-parallel: 6
       matrix:
-        python: ['3.8', '3.9', '3.10', '3.11', 'pypy3.10']
+        python: ['3.8', '3.9', '3.10', '3.11']
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3