fix(test): remove mock package test dependancy (#390)

Author: pndurette

Diff for: gtts/tests/test_tts.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 import os
 import pytest
-from mock import Mock
+from unittest.mock import Mock
 from six.moves import urllib
 
 from gtts.tts import gTTS, gTTSError

Diff for: pyproject.toml

@@ -46,7 +46,6 @@ tests = [
     "pytest ~= 7.1.3",
     "pytest-cov",
     "testfixtures",
-    "mock"
 ]
 docs = [
     "sphinx",