HTMLParseError was deprecated in Py3

There's a line attempting to import HTMLParseError here which results in schemato being unimportable in Python3:

In [1]: import schemato
INFO:rdflib:RDFLib Version: 4.2.1
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-3210cda68f48> in <module>()
----> 1 import schemato

/usr/local/lib/python3.5/dist-packages/schemato/__init__.py in <module>()
      1 from __future__ import absolute_import
      2 
----> 3 from .schemato import Schemato
      4 
      5 

/usr/local/lib/python3.5/dist-packages/schemato/schemato.py in <module>()
      8 
      9 from .compound_graph import CompoundGraph
---> 10 from .schemas.parselypage import ParselyPageValidator
     11 
     12 

/usr/local/lib/python3.5/dist-packages/schemato/schemas/parselypage.py in <module>()
      5 from six import StringIO, iteritems
      6 from six.moves.urllib.request import urlopen
----> 7 from six.moves.html_parser import HTMLParser, HTMLParseError
      8 
      9 from ..errors import _error

ImportError: cannot import name 'HTMLParseError'

This occurs because while six.moves makes html_parser available, the Python3 target no longer supports HTMLParseError.

It appears only to be raised from one function; probably a custom exception would suffice.

It's a small change but if you're short on time I'm happy to offer a PR?

Suggested change, which appears to work for me, is to use six to check for python version; if 2, import the old error. If 3, define a new exception. Client code is suggested to check for the schemato.schemas.parselypage.HTMLParseError in either case, rather than the html_parser error..but if they do in Py2, it's no big deal. In Py3 they'd never have written dependent code, because it's already broken.

Looks like this was only removed in the 3.5 release when they merged the patch here. You can see other people have worked around this by defining a custom class like in stephenmcd/mezzanine#1414. A PR would be much appreciated.