I'm not sure if you are seriously interested in iOS development, but most Macs in circulation that aren't Sierra compatible can be upgraded with this tool: http://dosdude1.com/sierrapatch.html

If you don't have the budget to buy a Sierra-compatible Mac (a few hundred dollars) You can also buy a used iPhone very cheaply, especially if you look for ones with damaged screens, and keep it on a version that's supported by pre-Sierra XCode.