In the past `Py_XDECREF()` was a macro. Preserve compat to ancient Python
versions by having a trampoline which calls `Py_XDECREF()`.
Fixes: #2043
Fixes: c0da36c4 ("Rage-cleanup.")
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
(cherry picked from commit 3299dd8fc6)