Use modern spelling for Thread.is_alive().

Bug: 203436762
Test: atest vts_kernel_net_tests
Change-Id: Ibb34f328e3c71f45a983ed09d7148058219a6df1
diff --git a/net/test/tun_twister.py b/net/test/tun_twister.py
index f42d789..2bd457e 100644
--- a/net/test/tun_twister.py
+++ b/net/test/tun_twister.py
@@ -98,7 +98,7 @@
     os.close(self._signal_write)
     self._thread.join(TunTwister._POLL_TIMEOUT_SEC)
     os.close(self._signal_read)
-    if self._thread.isAlive():
+    if self._thread.is_alive():
       raise RuntimeError("Timed out waiting for thread exit")
     # Re-raise any error thrown from our thread.
     if isinstance(self._error, Exception):