Explicitly register pytest markers to avoid warnings

mgorny · sethmlarson · commit ed850cff009b · 2021-01-21T08:16:53.000-06:00
diff --git a/setup.cfg b/setup.cfg
@@ -0,0 +1,4 @@
+[tool:pytest]
+addopts = --strict-markers
+markers =
+	slow: marks tests as slow (deselect with '-m "not slow"')
