Use unittest.mock instead of the mock package (#29323)

It's part of Python since 3.3:
https://docs.python.org/3/library/unittest.mock.html

Co-authored-by: jgraham <james@hoppipolla.co.uk>
diff --git a/tools/ci/tc/tests/test_decision.py b/tools/ci/tc/tests/test_decision.py
index 2f9018a..9190a90 100644
--- a/tools/ci/tc/tests/test_decision.py
+++ b/tools/ci/tc/tests/test_decision.py
@@ -1,4 +1,5 @@
-import mock
+from unittest import mock
+
 import pytest
 
 from tools.ci.tc import decision
diff --git a/tools/ci/tc/tests/test_valid.py b/tools/ci/tc/tests/test_valid.py
index 6db20a4..ff6ca7a 100644
--- a/tools/ci/tc/tests/test_valid.py
+++ b/tools/ci/tc/tests/test_valid.py
@@ -1,8 +1,8 @@
 import json
 import os
+from unittest import mock
 
 import jsone
-import mock
 import pytest
 import requests
 import yaml
diff --git a/tools/lint/tests/test_lint.py b/tools/lint/tests/test_lint.py
index e89499f..0fc21f7 100644
--- a/tools/lint/tests/test_lint.py
+++ b/tools/lint/tests/test_lint.py
@@ -1,8 +1,7 @@
 import io
 import os
 import sys
-
-import mock
+from unittest import mock
 
 from ...localpaths import repo_root
 from .. import lint as lint_mod
diff --git a/tools/lint/tests/test_path_lints.py b/tools/lint/tests/test_path_lints.py
index d9da56d..042c4f7 100644
--- a/tools/lint/tests/test_path_lints.py
+++ b/tools/lint/tests/test_path_lints.py
@@ -1,5 +1,5 @@
-import mock
 import os
+from unittest import mock
 
 from ..lint import check_path, check_unique_case_insensitive_paths
 from .base import check_errors
diff --git a/tools/manifest/tests/test_manifest.py b/tools/manifest/tests/test_manifest.py
index 654070f..6919381 100644
--- a/tools/manifest/tests/test_manifest.py
+++ b/tools/manifest/tests/test_manifest.py
@@ -1,6 +1,5 @@
 import os
-
-import mock
+from unittest import mock
 
 import hypothesis as h
 import hypothesis.strategies as hs
diff --git a/tools/manifest/tests/test_utils.py b/tools/manifest/tests/test_utils.py
index 1bdffb9..0d65a44 100644
--- a/tools/manifest/tests/test_utils.py
+++ b/tools/manifest/tests/test_utils.py
@@ -1,7 +1,6 @@
 import os
 import subprocess
-
-import mock
+from unittest import mock
 
 from .. import utils
 
diff --git a/tools/requirements_pytest.txt b/tools/requirements_pytest.txt
index 9f7680f..5d3831a 100644
--- a/tools/requirements_pytest.txt
+++ b/tools/requirements_pytest.txt
@@ -1,4 +1,3 @@
 pytest==6.2.4
 pytest-cov==2.12.1
-mock==4.0.3
 hypothesis==6.14.0
diff --git a/tools/wpt/tests/test_browser.py b/tools/wpt/tests/test_browser.py
index 591b028..19d3f83 100644
--- a/tools/wpt/tests/test_browser.py
+++ b/tools/wpt/tests/test_browser.py
@@ -1,9 +1,10 @@
 import logging
 import inspect
-import mock
-import pytest
 import subprocess
 import sys
+from unittest import mock
+
+import pytest
 
 from tools.wpt import browser
 
diff --git a/tools/wpt/tests/test_revlist.py b/tools/wpt/tests/test_revlist.py
index 30208f4..5823235 100644
--- a/tools/wpt/tests/test_revlist.py
+++ b/tools/wpt/tests/test_revlist.py
@@ -1,4 +1,5 @@
-import mock
+from unittest import mock
+
 from tools.wpt import revlist
 
 
diff --git a/tools/wpt/tests/test_run.py b/tools/wpt/tests/test_run.py
index d533977..d082928 100644
--- a/tools/wpt/tests/test_run.py
+++ b/tools/wpt/tests/test_run.py
@@ -1,7 +1,7 @@
-import mock
 import tempfile
 import shutil
 import sys
+from unittest import mock
 
 import pytest
 
diff --git a/tools/wpt/tests/test_testfiles.py b/tools/wpt/tests/test_testfiles.py
index b62975b..c9806c5 100644
--- a/tools/wpt/tests/test_testfiles.py
+++ b/tools/wpt/tests/test_testfiles.py
@@ -1,5 +1,5 @@
 import os.path
-from mock import patch
+from unittest.mock import patch
 
 from tools.manifest.manifest import Manifest
 from tools.wpt import testfiles
diff --git a/tools/wptrunner/wptrunner/tests/browsers/test_sauce.py b/tools/wptrunner/wptrunner/tests/browsers/test_sauce.py
index 9d0ffd9..3245d12 100644
--- a/tools/wptrunner/wptrunner/tests/browsers/test_sauce.py
+++ b/tools/wptrunner/wptrunner/tests/browsers/test_sauce.py
@@ -1,7 +1,7 @@
 import logging
 import sys
+from unittest import mock
 
-import mock
 import pytest
 
 from os.path import join, dirname
diff --git a/tools/wptrunner/wptrunner/tests/test_products.py b/tools/wptrunner/wptrunner/tests/test_products.py
index 1a38c00..120c7e7 100644
--- a/tools/wptrunner/wptrunner/tests/test_products.py
+++ b/tools/wptrunner/wptrunner/tests/test_products.py
@@ -1,6 +1,6 @@
 from os.path import join, dirname
+from unittest import mock
 
-import mock
 import pytest
 
 from .base import all_products, active_products
diff --git a/tools/wptrunner/wptrunner/tests/test_update.py b/tools/wptrunner/wptrunner/tests/test_update.py
index 7704383..848c6b5 100644
--- a/tools/wptrunner/wptrunner/tests/test_update.py
+++ b/tools/wptrunner/wptrunner/tests/test_update.py
@@ -1,9 +1,10 @@
 import json
-import mock
 import os
-import pytest
 import sys
 from io import BytesIO
+from unittest import mock
+
+import pytest
 
 from .. import metadata, manifestupdate
 from ..update.update import WPTUpdate
diff --git a/tools/wptrunner/wptrunner/tests/test_wpttest.py b/tools/wptrunner/wptrunner/tests/test_wpttest.py
index 9475454..59aa636 100644
--- a/tools/wptrunner/wptrunner/tests/test_wpttest.py
+++ b/tools/wptrunner/wptrunner/tests/test_wpttest.py
@@ -1,5 +1,5 @@
-import mock
 from io import BytesIO
+from unittest import mock
 
 from manifest import manifest as wptmanifest
 from manifest.item import TestharnessTest, RefTest
diff --git a/tools/wptserve/tests/test_request.py b/tools/wptserve/tests/test_request.py
index f69470d..f793a29 100644
--- a/tools/wptserve/tests/test_request.py
+++ b/tools/wptserve/tests/test_request.py
@@ -1,4 +1,4 @@
-import mock
+from unittest import mock
 
 from wptserve.request import Request, RequestHeaders, MultiDict
 
diff --git a/tools/wptserve/tests/test_response.py b/tools/wptserve/tests/test_response.py
index 16440da..34adc86 100644
--- a/tools/wptserve/tests/test_response.py
+++ b/tools/wptserve/tests/test_response.py
@@ -1,5 +1,5 @@
-import mock
 from io import BytesIO
+from unittest import mock
 
 from wptserve.response import Response