Add support for python2.

My development install has a backport for html.parser so it didn't
fail import but did in CI.

Author: rouilj

test/html_norm.py

@@ -18,7 +18,10 @@
 tests.
 
 """
-from html.parser import HTMLParser
+try:
+    from html.parser import HTMLParser
+except ImportError:
+    from HTMLParser import HTMLParser  # python2
 
 try:
     from htmlentitydefs import name2codepoint