Fix margin in sentinel and mock_open (GH-8188)

(cherry picked from commit f85af035)
Co-authored-by: Andrés Delfino <adelfino@gmail.com>

Doc/library/unittest.mock.rst

@@ -1825,12 +1825,12 @@ sentinel
 
 .. data:: sentinel
 
-    The ``sentinel`` object provides a convenient way of providing unique
-    objects for your tests.
+   The ``sentinel`` object provides a convenient way of providing unique
+   objects for your tests.
 
-    Attributes are created on demand when you access them by name. Accessing
-    the same attribute will always return the same object. The objects
-    returned have a sensible repr so that test failure messages are readable.
+   Attributes are created on demand when you access them by name. Accessing
+   the same attribute will always return the same object. The objects
+   returned have a sensible repr so that test failure messages are readable.
 
    .. versionchanged:: 3.7
       The ``sentinel`` attributes now preserve their identity when they are
@@ -2070,22 +2070,22 @@ mock_open
 
 .. function:: mock_open(mock=None, read_data=None)
 
-    A helper function to create a mock to replace the use of :func:`open`. It works
-    for :func:`open` called directly or used as a context manager.
-
-    The *mock* argument is the mock object to configure. If ``None`` (the
-    default) then a :class:`MagicMock` will be created for you, with the API limited
-    to methods or attributes available on standard file handles.
-
-    *read_data* is a string for the :meth:`~io.IOBase.read`,
-    :meth:`~io.IOBase.readline`, and :meth:`~io.IOBase.readlines` methods
-    of the file handle to return.  Calls to those methods will take data from
-    *read_data* until it is depleted.  The mock of these methods is pretty
-    simplistic: every time the *mock* is called, the *read_data* is rewound to
-    the start.  If you need more control over the data that you are feeding to
-    the tested code you will need to customize this mock for yourself.  When that
-    is insufficient, one of the in-memory filesystem packages on `PyPI
-    <https://pypi.org>`_ can offer a realistic filesystem for testing.
+   A helper function to create a mock to replace the use of :func:`open`. It works
+   for :func:`open` called directly or used as a context manager.
+
+   The *mock* argument is the mock object to configure. If ``None`` (the
+   default) then a :class:`MagicMock` will be created for you, with the API limited
+   to methods or attributes available on standard file handles.
+
+   *read_data* is a string for the :meth:`~io.IOBase.read`,
+   :meth:`~io.IOBase.readline`, and :meth:`~io.IOBase.readlines` methods
+   of the file handle to return.  Calls to those methods will take data from
+   *read_data* until it is depleted.  The mock of these methods is pretty
+   simplistic: every time the *mock* is called, the *read_data* is rewound to
+   the start.  If you need more control over the data that you are feeding to
+   the tested code you will need to customize this mock for yourself.  When that
+   is insufficient, one of the in-memory filesystem packages on `PyPI
+   <https://pypi.org>`_ can offer a realistic filesystem for testing.
 
    .. versionchanged:: 3.4
       Added :meth:`~io.IOBase.readline` and :meth:`~io.IOBase.readlines` support.