green selectors: Override DefaultSelector as well

The original patch[1] missed one thing which I just discovered - there's
selectors.DefaultSelector alias for the "the most efficient implementation
available on the current platform"[2].

Before this patch a non-green selector class would be obtained when
DefaultSelector was used.

[1] 0d509ef7d2
[2] https://docs.python.org/3.5/library/selectors.html#selectors.DefaultSelector

eventlet/green/selectors.py

@@ -4,6 +4,7 @@ from eventlet import patcher
 from eventlet.green import select
 
 __patched__ = [
+    'DefaultSelector',
     'SelectSelector',
 ]
 
@@ -29,3 +30,5 @@ del patcher
 
 if sys.platform != 'win32':
     SelectSelector._select = staticmethod(select.select)
+
+DefaultSelector = SelectSelector

tests/isolated/patcher_blocking_select_methods_are_deleted.py

@@ -24,4 +24,7 @@ if __name__ == '__main__':
         ]:
             assert not hasattr(selectors, name), name
 
+        default = selectors.DefaultSelector
+        assert default is selectors.SelectSelector, default
+
     print('pass')