mgr: Define PY_SSIZE_T_CLEAN ahead of every Python.h

mgr: Define PY_SSIZE_T_CLEAN ahead of every Python.h

Building on Fedora 35 with Python 3.10 makes vstart to loop
forever, throwing the following message:

 Error EINVAL: SystemError: PY_SSIZE_T_CLEAN macro must be
 defined for '#' formats

I followed the hint in the following document:
 https://docs.python.org/3/c-api/intro.html

It says "recommended" to always define PY_SSIZE_T_CLEAN,
but as you can see it is actually required in our case.

Fixes: https://tracker.ceph.com/issues/53441
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>