Use urllib.request.urlopen():

import urllib.request
with urllib.request.urlopen('http://www.example.com/') as f:
    html = f.read().decode('utf-8')

This is the most basic way to use the library, minus any error handling. You can also do more complex stuff such as changing headers.

On Python 2, the method is in urllib2:

import urllib2
response = urllib2.urlopen('http://www.example.com/')
html = response.read()