I had the old 10.6 SDK in /Developer/SDKs, it could have been ‘special’ in that I did alot of weird things for a former project and I just don’t recall.
For the sake of explanation though lets say it wasn’t, what then?I installed OSX Python 3.5.1, created a virtual environment using the pyvenv command from that newly installed Python 3.5.1, and sourced its bin/activate.Then when running pip install ZODB, the compiler would be passed the -isysroot flag for the 10.6 SDK, which would change its include paths and it would find no includes or libs
Not sure why that is, or what would make the virtual environment different, but there you go.