Fix 2 DeprecationWarnings about invalid escape seq

src/_pytest/python_api.py

@@ -497,7 +497,7 @@ def _is_numpy_array(obj):
 
 
 def raises(expected_exception, *args, **kwargs):
-    """
+    r"""
     Assert that a code block/function call raises ``expected_exception``
     and raise a failure exception otherwise.