Fix marshmallow warnings

[t-b]

commit 7e7a71ad7e2de50f9e1e5ae282bdf15c1973f358 (HEAD)
Author: Thomas Braun <thomas.braun@byte-physics.de>
Date:   Tue Aug 19 12:43:00 2025 +0200

    pytest.ini: Ignore marshmallow warnings
    
    Created #575 to fix them.

diff --git a/pytest.ini b/pytest.ini
index 626d70b..4fa723f 100644
--- a/pytest.ini
+++ b/pytest.ini
@@ -7,3 +7,5 @@ markers =
 
 filterwarnings =
     ignore:add_child is deprecated. Set the parent attribute instead.:DeprecationWarning
+    ignore::marshmallow.warnings.RemovedInMarshmallow4Warning
+    ignore::marshmallow.warnings.ChangedInMarshmallow4Warning