Thanks for your suggested patch and my apologies for not replying sooner.  For Python 2.7.15, we have done some housecleaning of old macOS support issues and, as part of ee8e4b61d6181f5bfadf6ee3d229f0ebda8a6ae9 for Issue32726, I have backported the deployment target setting behavior from 3.x, i.e. the deployment target on current systems is now set to the build system's OS level unless overridden by MACOSX_DEPLOYMENT_TARGET= on the configure command or as an environment variable.  So the previous behavior can be forced by:

./configure MACOSX_DEPLOYMENT_TARGET=10.4

As you noted, using a proper value for the deployment target allows things like using the system libedit readline support and standard library support for more recent system calls.