[Python-Dev] Drop support for AIX releases without dlopen

Python has supported using dynload_shlib (using dlopen) on AIX since https://github.com/python/cpython/commit/c19c5a62aef7dce0e8147655b0d2f087965... in 2003. Considering this is now 20 years later and all supported AIX versions support dlopen, I suspect nobody has used or tested this code path in quite some time. I propose removing this support under PEP 11. For reference, I opened https://bugs.python.org/issue42030 and changes were merged in https://github.com/python/cpython/pull/22717. I opened a PR to update PEP 11 in https://github.com/python/peps/issues/1653, but was told this needed discussion here. Sorry if this didn't quite follow the process correctly, I thought I had things in the right order, but I guess not.