You need to include brython_stdlib.js
in your html code. So your html should look like this:
<html>
<head>
<script src="https://stackoverflow.com/questions/40001634/src/Brython3.2.8/brython.js"></script>
<script src="../src/Brython3.2.8/brython_stdlib.js"></script>
</head>
<body onload="brython()">
<script type="text/python">
import sys
from browser import document, alert
def echo(ev):
alert(document["zone"].value)
document['mybutton'].bind('click', echo)
</script>
<input id="zone"><button id="mybutton">click !</button>
</body>
</html>