bpo-29512: Fix Lib/test/bisect.py shebang (#4522)

Replace python3 with python2.
diff --git a/Lib/test/bisect.py b/Lib/test/bisect.py
index 6fc5618..1bf32ef 100755
--- a/Lib/test/bisect.py
+++ b/Lib/test/bisect.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python2
 """
 Command line tool to bisect failing CPython tests.