diff --git a/python/mozbuild/mozpack/archive.py b/python/mozbuild/mozpack/archive.py
index 44fde17d84f5..06f5b4df7ac2 100644
--- a/python/mozbuild/mozpack/archive.py
+++ b/python/mozbuild/mozpack/archive.py
@@ -30,7 +30,10 @@ def create_tar_from_files(fp, files):
 
     FUTURE accept a filename argument (or create APIs to write files)
     """
-    with tarfile.open(name='', mode='w', fileobj=fp, dereference=True) as tf:
+    # The format is explicitly set to tarfile.GNU_FORMAT, because this default format
+    # has been changed in Python 3.8.
+    with tarfile.open(name='', mode='w', fileobj=fp, dereference=True,
+                      format=tarfile.GNU_FORMAT) as tf:
         for archive_path, f in sorted(files.items()):
             if not isinstance(f, BaseFile):
                 f = File(f)
diff --git a/taskcluster/taskgraph/test/test_util_docker.py b/taskcluster/taskgraph/test/test_util_docker.py
index e1ac0da3d34d..099fbb67f5ff 100644
--- a/taskcluster/taskgraph/test/test_util_docker.py
+++ b/taskcluster/taskgraph/test/test_util_docker.py
@@ -7,12 +7,10 @@ from __future__ import absolute_import, print_function, unicode_literals
 import os
 import shutil
 import stat
-import sys
 import tarfile
 import tempfile
 import unittest
 import mock
-import pytest
 import taskcluster_urls as liburls
 
 from taskgraph.util import docker
@@ -24,10 +22,6 @@ MODE_STANDARD = stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH
 
 @mock.patch.dict('os.environ', {'TASKCLUSTER_ROOT_URL': liburls.test_root_url()})
 class TestDocker(unittest.TestCase):
-
-    @pytest.mark.xfail(
-        sys.version_info >= (3, 0), reason="python3 migration is not complete"
-    )
     def test_generate_context_hash(self):
         tmpdir = tempfile.mkdtemp()
         try:
@@ -81,9 +75,6 @@ class TestDocker(unittest.TestCase):
         with MockedOpen(files):
             self.assertEqual(docker.docker_image('myimage', by_tag=True), "mozilla/myimage:1.2.3")
 
-    @pytest.mark.xfail(
-        sys.version_info >= (3, 0), reason="python3 migration is not complete"
-    )
     def test_create_context_tar_basic(self):
         tmp = tempfile.mkdtemp()
         try:
@@ -110,9 +101,6 @@ class TestDocker(unittest.TestCase):
         finally:
             shutil.rmtree(tmp)
 
-    @pytest.mark.xfail(
-        sys.version_info >= (3, 0), reason="python3 migration is not complete"
-    )
     def test_create_context_topsrcdir_files(self):
         tmp = tempfile.mkdtemp()
         try:
@@ -183,9 +171,6 @@ class TestDocker(unittest.TestCase):
         finally:
             shutil.rmtree(tmp)
 
-    @pytest.mark.xfail(
-        sys.version_info >= (3, 0), reason="python3 migration is not complete"
-    )
     def test_create_context_extra_directory(self):
         tmp = tempfile.mkdtemp()
         try: