Improve highlighting of some code blocks. (GH-6401)

(cherry picked from commit 46936d5a71d1683dbd8ddb6d7f39aab50ecfec50)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Doc/library/timeit.rst b/Doc/library/timeit.rst
index 8bbadf1..d922fd6 100644
--- a/Doc/library/timeit.rst
+++ b/Doc/library/timeit.rst
@@ -25,7 +25,7 @@
 The following example shows how the :ref:`timeit-command-line-interface`
 can be used to compare three different expressions:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 -m timeit '"-".join(str(n) for n in range(100))'
    10000 loops, best of 5: 30.2 usec per loop
@@ -264,7 +264,7 @@
 
 It is possible to provide a setup statement that is executed only once at the beginning:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python -m timeit -s 'text = "sample string"; char = "g"'  'char in text'
    5000000 loops, best of 5: 0.0877 usec per loop
@@ -293,7 +293,7 @@
 Here we compare the cost of using :func:`hasattr` vs. :keyword:`try`/:keyword:`except`
 to test for missing and present object attributes:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python -m timeit 'try:' '  str.__bool__' 'except AttributeError:' '  pass'
    20000 loops, best of 5: 15.7 usec per loop