fix wdb on gevent/eventlet on PY2

Here's a test case:

```python
import gevent
import gevent.monkey
gevent.monkey.patch_all()
import wsgiref.simple_server
import wdb.ext
wsgiref.simple_server.make_server('127.0.0.1', 8300, wdb.ext.WdbMiddleware(wsgiref.simple_server.demo_app)).serve_forever()
```

To test, run wdb.server.py in the background and also run this with WDB_NO_BROWSER_AUTO_OPEN=1 and visit http://localhost:8300/__wdb/shell.

With gevent>=1.0 on Python 2 it will spew errors, but with this patch (that uses `socket.socket` which is monkey-patched to `gevent.socket.socket`) it works perfectly.