Drop dependency of unittest2; use stdlib unittest instead

Now that Python 2.6 support has been dropped (commit
b67fe8a6f55ba321b602740fdf0bf0d15f074f4f), can simply use Python
unittest. No unique features of unittest2 were in use that aren't also
available in the stdlib unittest. One less dependency.
diff --git a/docs/index.txt b/docs/index.txt
index 6c8e37b..9f6e3c2 100644
--- a/docs/index.txt
+++ b/docs/index.txt
@@ -90,7 +90,7 @@
 Bug Reports
 +++++++++++
 
-Mock uses `unittest2 <http://pypi.python.org/pypi/unittest2>`_ for its own
+Mock uses `unittest <https://docs.python.org/3/library/unittest.html>`_ for its own
 Issues with the backport process, such as compatibility with a particular
 Python, should be reported to the `bug tracker
 <https://github.com/testing-cabal/mock/issues>`_. Feature requests and issues