Replace backquote with command substitution in subprocess doc example (GH-13941)

Replace backquotes with POSIXy command substitution in example.

Doc/library/subprocess.rst

@@ -1166,12 +1166,12 @@ In the following examples, we assume that the relevant functions have already
 been imported from the :mod:`subprocess` module.
 
 
-Replacing /bin/sh shell backquote
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Replacing :program:`/bin/sh` shell command substitution
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 .. code-block:: bash
 
-   output=`mycmd myarg`
+   output=$(mycmd myarg)
 
 becomes::
 
@@ -1182,7 +1182,7 @@ Replacing shell pipeline
 
 .. code-block:: bash
 
-   output=`dmesg | grep hda`
+   output=$(dmesg | grep hda)
 
 becomes::
 
@@ -1199,7 +1199,7 @@ be used directly:
 
 .. code-block:: bash
 
-   output=`dmesg | grep hda`
+   output=$(dmesg | grep hda)
 
 becomes::
 

Doc/tools/susp-ignored.csv

@@ -218,8 +218,6 @@ library/stdtypes,,:len,s[len(s):len(s)]
 library/stdtypes,,::,>>> y = m[::2]
 library/stdtypes,,::,>>> z = y[::-2]
 library/string,,`,"!""#$%&'()*+,-./:;<=>?@[\]^_`{|}~"
-library/subprocess,,`,"output=`dmesg | grep hda`"
-library/subprocess,,`,"output=`mycmd myarg`"
 library/tarfile,,:bz2,
 library/tarfile,,:compression,filemode[:compression]
 library/tarfile,,:gz,