Fix a crash in pypy.

Apparently, in pypy, "signal.set_wakeup_fd" doesn't have the keyword argument warn_on_full_buffer.

Here is a simple script to reproduce this problem:
import trio

async def handler(stream: trio.abc.Stream):
    await stream.receive_some(1)

async def trio_main():
    socket_listeners = await trio.serve_tcp(handler)

trio.run(trio_main)

trio/_core/_wakeup_socketpair.py

@@ -6,7 +6,7 @@
 from .. import _core
 from .._util import is_main_thread
 
-if sys.version_info >= (3, 7):
+if sys.version_info >= (3, 7) and '__pypy__' not in sys.builtin_module_names:
     HAVE_WARN_ON_FULL_BUFFER = True
 else:
     HAVE_WARN_ON_FULL_BUFFER = False