[KevinH]

I added a Current Version: "0.3.4" line to my ePub3-itizer release page.

Then you could simplify things using regular expressions to be just:

from __future__ import unicode_literals, division, absolute_import, print_function

import sys
import os
import re

try:
    import urllib.request as url_request
except ImportError:
    import urllib2 as url_request


_latest_pattern = re.compile(r'Current Version:\s*"([^&]*)&')
_version_pattern = re.compile(r'<version>([^<]*)</version>')

def check_for_updates(site_url, plugin_path):
    latest_version = None
    installed_version = None

    try:
        # get the latest version from the plugin release website page
        req = url_request.Request(site_url)
        response = url_request.urlopen(req)
        the_page = response.read()
        the_page = the_page.decode('utf-8')
        m = _latest_pattern.search(the_page)
        if m:
            latest_version = (m.group(1).strip())

        # get installed version of plugin
        ppath = os.path.join(plugin_path, "plugin.xml")
        with open(ppath,'rb') as f:
            data = f.read()
            data = data.decode('utf-8')
            m = _version_pattern.search(data)
            if m:
                installed_version = m.group(1).strip()
    except:
        pass

    return latest_version, installed_version


SITE_URL = "https://www.mobileread.com/forums/showpost.php?p=2973066&postcount=1"
PLUGIN_PATH = "/Users/kbhend/Desktop"
latest_version, installed_version = check_for_updates(SITE_URL, PLUGIN_PATH)
print(latest_version, installed_version)