socketpair is not an __extension__ on python 3 because it doesn't come from socketcommon.

src/gevent/_socket3.py

@@ -713,8 +713,6 @@ else:
             # was successfully imported, leading to subsequent imports to cause
             # ValueError
             __implements__.remove('socketpair')
-        if 'socketpair' not in __extensions__:
-            __extensions__.append('socketpair')
 
 
 # PyPy needs drop and reuse