bpo-11233: Create availability directive for documentation (GH-9692)

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

Co-Authored-By: Georg Brandl <georg@python.org>
diff --git a/Doc/tools/extensions/pyspecific.py b/Doc/tools/extensions/pyspecific.py
index 8036daa..132920f 100644
--- a/Doc/tools/extensions/pyspecific.py
+++ b/Doc/tools/extensions/pyspecific.py
@@ -131,6 +131,25 @@
         return [pnode]
 
 
+# Support for documenting platform availability
+
+class Availability(Directive):
+
+    has_content = False
+    required_arguments = 1
+    optional_arguments = 0
+    final_argument_whitespace = True
+
+    def run(self):
+        pnode = nodes.paragraph(classes=['availability'])
+        n, m = self.state.inline_text(':ref:`Availability <availability>`: ',
+                                      self.lineno)
+        pnode.extend(n + m)
+        n, m = self.state.inline_text(self.arguments[0], self.lineno)
+        pnode.extend(n + m)
+        return [pnode]
+
+
 # Support for documenting decorators
 
 class PyDecoratorMixin(object):
@@ -401,6 +420,7 @@
     app.add_role('issue', issue_role)
     app.add_role('source', source_role)
     app.add_directive('impl-detail', ImplementationDetail)
+    app.add_directive('availability', Availability)
     app.add_directive('deprecated-removed', DeprecatedRemoved)
     app.add_builder(PydocTopicsBuilder)
     app.add_builder(suspicious.CheckSuspiciousMarkupBuilder)