tests/extmod: Skip uselect_poll_udp when poll() is not available.

This is the same fix as applied in uselect_poll_basic.py.

tests/extmod/uselect_poll_udp.py

@@ -5,7 +5,9 @@
 except ImportError:
     try:
         import socket, select
-    except ImportError:
+
+        select.poll  # Raises AttributeError for CPython implementations without poll()
+    except (ImportError, AttributeError):
         print("SKIP")
         raise SystemExit