Explicitly declared the "slow" pytest marker

This should make the latest pytest happy.

exceptiongroup/tests/conftest.py

@@ -5,6 +5,10 @@ def pytest_addoption(parser):
     parser.addoption("--run-slow", action="store_true", help="run slow tests")
 
 
+def pytest_configure(config):
+    config.addinivalue_line("markers", "slow: mark a time consuming test")
+
+
 def pytest_collection_modifyitems(config, items):
     if config.getoption("--run-slow", True):
         # --runslow given in cli: do not skip slow tests