socket on Windows: do not provide socketpair

to fix this exception:

  Traceback (most recent call last):
    File "test__local.py", line 4, in <module>
      from gevent import monkey; monkey.patch_all()
    File "c:\projects\gevent\gevent\monkey.py", line 369, in patch_all
      patch_socket(dns=dns, aggressive=aggressive)
    File "c:\projects\gevent\gevent\monkey.py", line 276, in patch_socket
      patch_module('socket', items=items)
    File "c:\projects\gevent\gevent\monkey.py", line 102, in patch_module
      patch_item(module, attr, getattr(gevent_module, attr))
  AttributeError: 'module' object has no attribute 'socketpair'

gevent/_socket3.py

@@ -447,6 +447,9 @@ if hasattr(_socket, "socketpair"):
         b = socket(family, type, proto, b.detach())
         return a, b
 
+else:
+    __implements__.remove('socketpair')
+
 
 # PyPy needs drop and reuse
 def _do_reuse_or_drop(socket, methname):