diff --git a/pytest.ini b/pytest.ini
--- a/pytest.ini
+++ b/pytest.ini
@@ -1,3 +1,5 @@
 [pytest]
 addopts = --doctest-modules
 norecursedirs = docs
+markers =
+    fs: tests that involve filesystem ios
diff --git a/swh/model/merkle.py b/swh/model/merkle.py
--- a/swh/model/merkle.py
+++ b/swh/model/merkle.py
@@ -6,7 +6,7 @@
 """Merkle tree data structure"""
 
 import abc
-import collections
+from collections.abc import Mapping
 
 from typing import Iterator, List, Set
 
@@ -66,7 +66,7 @@
 
     """
     for key, rvalue in right.items():
-        if isinstance(rvalue, collections.Mapping):
+        if isinstance(rvalue, Mapping):
             new_lvalue = deep_update(left.get(key, {}), rvalue)
             left[key] = new_lvalue
         else:
diff --git a/swh/model/tests/test_model.py b/swh/model/tests/test_model.py
--- a/swh/model/tests/test_model.py
+++ b/swh/model/tests/test_model.py
@@ -456,7 +456,7 @@
     extra_headers = (
         (b"header1", b"value1"),
         (b"header2", b"42"),
-        (b"header3", b"should I?\u0000"),
+        (b"header3", b"should I?\x00"),
         (b"header1", b"again"),
     )
 
@@ -479,7 +479,7 @@
     extra_headers = (
         (b"header1", b"value1"),
         (b"header2", b"42"),
-        (b"header3", b"should I?\u0000"),
+        (b"header3", b"should I?\x00"),
         (b"header1", b"again"),
     )
 
@@ -501,7 +501,7 @@
     extra_headers = (
         (b"header1", b"value1"),
         (b"header2", b"42"),
-        (b"header3", b"should I?\u0000"),
+        (b"header3", b"should I?\x00"),
         (b"header1", b"again"),
     )