BUG/MEDIUM: spoa/python: Fixing references to None

BUG/MEDIUM: spoa/python: Fixing references to None

As per https://docs.python.org/3/c-api/none.html, None has to be treated
exactly like other objects for reference counting.
So, when we use it, we need to INCREF and when we are done, DECREF

This patch must be backported as far as 2.0.

(cherry picked from commit bdd034947c4a3a6f8c41af9bf6e62b7a74b4ebbc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 30189a6e056799f38d57478b04baded0868a431b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 752e24a9e42cd9fe76f6b0d070562dc180c92555)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>