Use xml.sax.saxutils.escape instead of deprecated cgi.escape

[messense]

/usr/local/lib/python3.6/dist-packages/mammoth/writers/html.py:34: DeprecationWarning: cgi.escape is deprecated, use html.escape instead
  return cgi.escape(text, quote=True)

Ref: Escaping HTML on Python wiki.

[mwilliamson]

Thanks for the suggestion. A couple of thoughts:

• This introduces an inconsistency between Python 2 and 3 due to the different handling of single quotes
• Importing on every function call rather than on module load seems unnecessary

[messense]

Changed to use xml.sax.saxutils.escape instead of html.escape since it exists on both Python 2 & 3.

[taleinat]

Note that in Python 3.8 cgi.escape has been removed, so fixing this is required to support 3.8.

[taleinat]

Also, why not:

try:
    from html import escape as html_escape
except ImportError:
    from cgi import escape as html_escape

[mwilliamson]

Python 3.8 support should be fixed in the latest version. Thanks for the pull request!