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

diff --git a/Doc/howto/argparse.rst b/Doc/howto/argparse.rst
index 9d770f5..e78a022 100644
--- a/Doc/howto/argparse.rst
+++ b/Doc/howto/argparse.rst
@@ -24,7 +24,7 @@
 Let's show the sort of functionality that we are going to explore in this
 introductory tutorial by making use of the :command:`ls` command:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ ls
    cpython  devguide  prog.py  pypy  rm-unused-function.patch
@@ -77,7 +77,7 @@
 
 Following is a result of running the code:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py
    $ python3 prog.py --help
@@ -119,7 +119,7 @@
 
 And running the code:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py
    usage: prog.py [-h] echo
@@ -164,7 +164,7 @@
 
 And we get:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py -h
    usage: prog.py [-h] echo
@@ -185,7 +185,7 @@
 
 Following is a result of running the code:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4
    Traceback (most recent call last):
@@ -206,7 +206,7 @@
 
 Following is a result of running the code:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4
    16
@@ -233,7 +233,7 @@
 
 And the output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py --verbosity 1
    verbosity turned on
@@ -279,7 +279,7 @@
 
 And the output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py --verbose
    verbosity turned on
@@ -325,7 +325,7 @@
 
 And here goes:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py -v
    verbosity turned on
@@ -359,7 +359,7 @@
 
 And now the output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py
    usage: prog.py [-h] [-v] square
@@ -395,7 +395,7 @@
 
 And the output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4
    16
@@ -429,7 +429,7 @@
 
 And the output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4 -v 3
    usage: prog.py [-h] [-v {0,1,2}] square
@@ -470,7 +470,7 @@
 We have introduced another action, "count",
 to count the number of occurrences of a specific optional arguments:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4
    16
@@ -537,7 +537,7 @@
 
 And this is what it gives:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4 -vvv
    the square of 4 equals 16
@@ -581,7 +581,7 @@
 
 And:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4
    16
@@ -614,7 +614,7 @@
 
 Output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py
    usage: prog.py [-h] [-v] x y
@@ -652,7 +652,7 @@
 
 Output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4 2
    16
@@ -695,7 +695,7 @@
 Our program is now simpler, and we've lost some functionality for the sake of
 demonstration. Anyways, here's the output:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py 4 2
    4^2 == 16
@@ -739,7 +739,7 @@
 which tells us that we can either use ``-v`` or ``-q``,
 but not both at the same time:
 
-.. code-block:: sh
+.. code-block:: shell-session
 
    $ python3 prog.py --help
    usage: prog.py [-h] [-v | -q] x y