Add type annotation

Author: vkucera

Scripts/o2_linter.py

@@ -465,7 +465,7 @@ class TestPdgExplicitMass(TestSpec):
     name = "pdg/explicit-mass"
     message = "Avoid hard-coded particle masses. Use o2::constants::physics::Mass... instead."
     suffixes = [".h", ".cxx"]
-    masses = []  # list of mass values to detect
+    masses: "list[str]" = []  # list of mass values to detect
 
     def __init__(self) -> None:
         super().__init__()