[3.7] bpo-11233: Create availability directive for documentation (GH-9692) (GH-9830)

Replace "Availability: xxx" with ".. availability:: xxx" in the doc.
 Original patch by Georg Brandl.

 Co-Authored-By: Georg Brandl <georg@python.org>
 (cherry picked from commit 2d6097d027e0dd3debbabc702aa9c98d94ba32a3)

 Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 23beb52..f22f224 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -1330,7 +1330,7 @@
       >>> subprocess.getstatusoutput('/bin/kill $$')
       (-15, '')
 
-   Availability: POSIX & Windows
+   .. availability:: POSIX & Windows.
 
    .. versionchanged:: 3.3.4
       Windows support was added.
@@ -1350,7 +1350,7 @@
       >>> subprocess.getoutput('ls /bin/ls')
       '/bin/ls'
 
-   Availability: POSIX & Windows
+   .. availability:: POSIX & Windows.
 
    .. versionchanged:: 3.3.4
       Windows support added