Issue #17128: Use private version of OpenSSL for 3.x OS X 10.5+ installers.

Issue #17128: Use private version of OpenSSL for 3.x OS X 10.5+ installers.
Among other issues, the Apple-supplied 0.9.7 libs for the 10.5 ABI cannot
verify newer SHA-256 certs as now used by python.org services.  Document
in the installer ReadMe some of the certificate management issues that
users now need to be more concerned with due to PEP 476's enabling cert
verification by default.  For now, continue to use the Apple-supplied
0.9.8 libs for the 10.6+ installer since they use Apple private APIs to
verify certificates using the system- and user-managed CA keychain stores.