Treat all warnings as errors during tests

This helps catching incorrect/deprecated API usage of third-party
libraries and also potentials ressources not properly disposed during
tests (for example, Pytest reports a warking internally if a file object
wasn't closed).

pyproject.toml

@@ -9,6 +9,8 @@ strict = true
 [tool.pytest.ini_options]
 addopts = "-l"
 filterwarnings = [
+  # By default all warnings are treated as errors.
+  'error',
   # Mixing threads and "fork()" is deprecated, but we need to test it anyway.
   'ignore:.*use of fork\(\) may lead to deadlocks in the child.*:DeprecationWarning',
   # The "dateutil" package used by "freezegun" relies on some deprecated APIs.