Don't share the settings fixture between tests.

tests/conftest.py

@@ -8,7 +8,7 @@ from sigal.settings import read_settings
 CURRENT_DIR = os.path.abspath(os.path.dirname(__file__))
 
 
-@pytest.fixture(scope='session')
+@pytest.fixture
 def settings():
     """Read the sample config file."""
     return read_settings(os.path.join(CURRENT_DIR, 'sample', 'sigal.conf.py'))