diff --git a/nbformat/sign.py b/nbformat/sign.py
index edb2c42b..572e10fc 100644
--- a/nbformat/sign.py
+++ b/nbformat/sign.py
@@ -602,7 +602,7 @@ def sign_notebook_file(self, notebook_path):
         if not os.path.exists(notebook_path):
             self.log.error("Notebook missing: %s" % notebook_path)
             self.exit(1)
-        with open(notebook_path, encoding="utf8") as f:
+        with open(notebook_path, encoding="utf-8") as f:
             nb = read(f, NO_CONVERT)
         self.sign_notebook(nb, notebook_path)
 
diff --git a/nbformat/validator.py b/nbformat/validator.py
index d578eb18..f72fbbcb 100644
--- a/nbformat/validator.py
+++ b/nbformat/validator.py
@@ -99,7 +99,7 @@ def _get_schema_json(v, version=None, version_minor=None):
     else:
         msg = "Cannot find appropriate nbformat schema file."
         raise AttributeError(msg)
-    with open(schema_path) as f:
+    with open(schema_path, encoding='utf-8') as f:
         schema_json = json.load(f)
     return schema_json
 
diff --git a/pyproject.toml b/pyproject.toml
index 6ed30ff0..ac5c0140 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -69,7 +69,7 @@ build = "make -C docs html SPHINXOPTS='-W'"
 [tool.hatch.envs.test]
 features = ["test"]
 [tool.hatch.envs.test.scripts]
-test = "python -m pytest -vv {args}"
+test = "PYTHONWARNDEFAULTENCODING=1 python -m pytest -vv {args}"
 nowarn = "test -W default {args}"
 
 [tool.hatch.envs.cov]
diff --git a/tests/test_sign.py b/tests/test_sign.py
index 59724eb8..97d8ad8f 100644
--- a/tests/test_sign.py
+++ b/tests/test_sign.py
@@ -40,7 +40,7 @@ def tearDown(self):
 
     def test_invalid_db_file(self):
         invalid_sql_file = os.path.join(self.data_dir, "invalid_db_file.db")
-        with open(invalid_sql_file, "w") as tempfile:
+        with open(invalid_sql_file, "w", encoding="utf-8") as tempfile:
             tempfile.write("[invalid data]")
 
         invalid_notary = sign.NotebookNotary(
diff --git a/tests/v4/test_json.py b/tests/v4/test_json.py
index 30c1eec6..21c7f9e9 100644
--- a/tests/v4/test_json.py
+++ b/tests/v4/test_json.py
@@ -108,7 +108,9 @@ def test_latest_schema_matches(self):
 
     def test_base_version_matches_latest(self):
         """Test to ensure latest version file matches latest verison"""
-        with open(os.path.join(BASE_PATH, "nbformat.v4.schema.json")) as schema_file:
+        with open(
+            os.path.join(BASE_PATH, "nbformat.v4.schema.json"), encoding='utf-8'
+        ) as schema_file:
             latest_schema = json.load(schema_file)
             with open(
                 os.path.join(
@@ -117,13 +119,16 @@ def test_base_version_matches_latest(self):
                         major=nbformat, minor=nbformat_minor
                     ),
                 ),
+                encoding='utf-8',
             ) as schema_file:  # noqa
                 ver_schema = json.load(schema_file)
             assert latest_schema == ver_schema
 
     def test_latest_matches_nbformat(self):
         """Test to ensure that the nbformat version matches the description of the latest schema"""
-        with open(os.path.join(BASE_PATH, "nbformat.v4.schema.json")) as schema_file:
+        with open(
+            os.path.join(BASE_PATH, "nbformat.v4.schema.json"), encoding='utf-8'
+        ) as schema_file:
             schema = json.load(schema_file)
         assert schema["description"] == "Jupyter Notebook v{major}.{minor} JSON schema.".format(
             major=nbformat, minor=nbformat_minor