Replace PyPI mock with unittest.mock (#111)

[Contributed by @musicinmybrain, reviewed by Lydia]

The standard library has contained unittest.mock since Python 3.3.

See also https://fedoraproject.org/wiki/Changes/DeprecatePythonMock for
details and context.

Author: lydia-duncan

test-requirements.txt

@@ -1,5 +1,4 @@
 flake8==7.3.0
-mock
 pytest==8.4.1
 pytest-cov
 tox

test/test_chapeldomain.py

@@ -5,8 +5,8 @@
 from __future__ import print_function, unicode_literals
 
 import docutils.nodes as nodes
-import mock
 import unittest
+from unittest import mock
 
 from sphinxcontrib.chapeldomain import (
     ChapelDomain, ChapelModuleIndex, ChapelClassObject, ChapelModuleLevel, ChapelObject,