gh-154757: List the 3.16 C API deprecations on the Deprecations page

The C API section of Doc/deprecations/index.rst started at
c-api-pending-removal-in-3.18.rst, so the 26 APIs pending removal in 3.16 were
not listed there. They are still present in Include/, so the page should show
them while they are pending.

Author: fedonman

Doc/deprecations/index.rst

@@ -18,6 +18,8 @@ Deprecations
 C API deprecations
 ------------------
 
+.. include:: c-api-pending-removal-in-3.16.rst
+
 .. include:: c-api-pending-removal-in-3.18.rst
 
 .. include:: c-api-pending-removal-in-3.19.rst