Run pytest under stricter conditions

This enables more errors and warnings, which should prevent regressing
on problems that have already been fixed.

I tried to also add `-X warn_default_encoding`, but `regex` (required by
`parsimonious`) fails with it enabled.

Author: aphedges

.github/workflows/test-pr.yml

@@ -43,4 +43,4 @@ jobs:
           pip install -e .[dev]
       - name: Test
         run: |
-          pytest
+          python -X dev -bb -m pytest