Add an explicit marker for pytest-timeout (#6054)

To avoid "'timeout' not found in  configuration option"
When using --strict-markers with pytest locally.

Author: Pierre-Sassoulas

setup.cfg

@@ -85,6 +85,7 @@ markers =
     primer_external_batch_one: Checks for crashes and errors when running pylint on external libs (batch one)
     primer_external_batch_two: Checks for crashes and errors when running pylint on external libs (batch two)
     benchmark: Baseline of pylint performance, if this regress something serious happened
+    timeout: Marks from pytest-timeout.
 
 [isort]
 profile = black