[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/os.path.rst b/Doc/library/os.path.rst
index 36bb21c..75e7e5b 100644
--- a/Doc/library/os.path.rst
+++ b/Doc/library/os.path.rst
@@ -85,7 +85,7 @@
    pathnames, or if *paths* is empty.  Unlike :func:`commonprefix`, this
    returns a valid path.
 
-   Availability: Unix, Windows
+   .. availability:: Unix, Windows.
 
    .. versionadded:: 3.5
 
@@ -349,7 +349,7 @@
 
    *start* defaults to :attr:`os.curdir`.
 
-   Availability: Unix, Windows.
+   .. availability:: Unix, Windows.
 
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
@@ -361,7 +361,7 @@
    This is determined by the device number and i-node number and raises an
    exception if an :func:`os.stat` call on either pathname fails.
 
-   Availability: Unix, Windows.
+   .. availability:: Unix, Windows.
 
    .. versionchanged:: 3.2
       Added Windows support.
@@ -377,7 +377,7 @@
 
    Return ``True`` if the file descriptors *fp1* and *fp2* refer to the same file.
 
-   Availability: Unix, Windows.
+   .. availability:: Unix, Windows.
 
    .. versionchanged:: 3.2
       Added Windows support.
@@ -393,7 +393,7 @@
    :func:`os.lstat`, or :func:`os.stat`.  This function implements the
    underlying comparison used by :func:`samefile` and :func:`sameopenfile`.
 
-   Availability: Unix, Windows.
+   .. availability:: Unix, Windows.
 
    .. versionchanged:: 3.4
       Added Windows support.