That's very odd!  I don't have a SuSE system available to test with but it is not clear to me why that would have different behavior because where the standard library extension modules directory (lib-dynload) is installed is controlled by the Python configure and Makefile and, in the vanilla copies downloadable from python.org, I can't find any relevant references to lib64 (there are some in setup.py as far as where to look for linking with other library files).  Are you sure you are using an unpatched 3.5.1 source tarball downloaded from python.org?  And that any lib-dynload in /lib64 wasn't installed by SuSE?  When I follow the steps you give, I end up with the extension modules in ~/.opt/lib/python3.5/lib-dynload as expected.  I don't know what else to suggest.  Anyone else have any ideas?