OK, I found this gitHub repo with all the old SDKs:

https://github.com/phracker/MacOSX-SDKs

I put a copy of the 10.6 SDk in it's old location:

/Developer/SDKs/MacOSX10.6.sdk/

/before I did that, I did a pip install of zodb in a virtualenv, and all seemed to go fine. THe extensions rae linked against :

$ otool -L cPersistence.cpython-35m-darwin.so
cPersistence.cpython-35m-darwin.so:
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1197.1.1)

where that comes from -- who knows??

then I tried again, after putting the sdk in place:

[note: serious pain the $%^# to get pip to stop using it's cache! ]

and everything seemed to build fine, and at least one simple test of zodb works fine. And this is how the extensions are linked:

$  otool -L cPersistence.cpython-35m-darwin.so
cPersistence.cpython-35m-darwin.so:
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.11)

which is the same as above, but the "current version" is much older -- so does that mean it used the 10.6 SDK????

Anyway, I can't reproduce the OP's bug.....

-CHB