FYI: after manually creating the .../libpython3.5/config directory and copying three files (see below) I was able to "pip3 install cython" as test.

 At the packing area:
michael@x071:[/data/prj/python3/python3-3.5.2]ls Xany/opt/lib/python3.5/config
ld_so_aix   makexp_aix  python.exp

in the package:
michael@x071:[/data/prj/python3/python3-3.5.2]restore -Tqf installp/ppc/*.I | grep lib/python3.5/config
New volume on installp/ppc/aixtools.python3.3.5.2.0.I:
Cluster size is 51200 bytes (100 blocks).
The volume number is 1.
The backup date is: Wed Nov 16 21:48:37 UTC 2016
Files are backed up by name.
The user is root.
./opt/lib/python3.5/config
./opt/lib/python3.5/config/ld_so_aix
./opt/lib/python3.5/config/python.exp
./opt/lib/python3.5/config/makexp_aix
./opt/lib/python3.5/configparser.py
./opt/lib/python3.5/config-3.5m
./opt/lib/python3.5/config-3.5m/libpython3.5m.a
./opt/lib/python3.5/config-3.5m/Setup
./opt/lib/python3.5/config-3.5m/python-config.py
./opt/lib/python3.5/config-3.5m/config.c
./opt/lib/python3.5/config-3.5m/Setup.local
./opt/lib/python3.5/config-3.5m/config.c.in
./opt/lib/python3.5/config-3.5m/makesetup
./opt/lib/python3.5/config-3.5m/Setup.config
./opt/lib/python3.5/config-3.5m/install-sh
./opt/lib/python3.5/config-3.5m/Makefile
./opt/lib/python3.5/config-3.5m/python.o
The number of archived files is 7390.

Results:

+-----------------------------------------------------------------------------+
                         Installing Software...
+-----------------------------------------------------------------------------+

installp:  APPLYING software for:
        aixtools.python3.rte 3.5.2.0

Restoring files, please wait.
4250 files restored.
Finished processing all filesets.  (Total time:  1 mins 1 secs).

+-----------------------------------------------------------------------------+
                                Summaries:
+-----------------------------------------------------------------------------+

Installation Summary
--------------------
Name                        Level           Part        Event       Result
-------------------------------------------------------------------------------
aixtools.python3.rte        3.5.2.0         USR         APPLY       SUCCESS

root@x064:[/data/prj/python3/python3-3.5.2]export OBJECT_MODE=64

root@x064:[/data/prj/python3/python3-3.5.2]pip3 install cython
Collecting cython
  Using cached Cython-0.25.1.tar.gz
Installing collected packages: cython
  Running setup.py install for cython ... done
Successfully installed cython-0.25.1
You are using pip version 8.1.1, however version 9.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

Question: if there is a patch since 3.5.2 was released - how do I pick this up in order to test (and maybe patch)? I am a bit lost in the hg/git discussion about where to go to get current status.