Run tests in serial again

Because this didn't fix the issue I'm seeing in #1802

Revert "Run tests in serial, refs #1802"

This reverts commit b91e17280c.

.github/workflows/test.yml

@@ -33,7 +33,8 @@ jobs:
         pip freeze
     - name: Run tests
       run: |
-        pytest
+        pytest -n auto -m "not serial"
+        pytest -m "serial"
     - name: Check if cog needs to be run
       run: |
         cog --check docs/*.rst