HTTP Error 403: Forbidden w/ User Agents Added

Usually I've been able to get around 403 Errors once I've added a known User Agent but I'm now trying to login and then eventually scrape and cannot figure out how to bypass this error.

Code:

import urllib
import http.cookiejar

cj = http.cookiejar.CookieJar()
opener = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(cj))
opener.addheaders = [('User-agent', 'Mozilla/5.0')]
urllib.request.install_opener(opener)
authentication_url = 'http://ift.tt/iMnaLP'
payload = {
    'session_key': 'email',
    'session_password': 'password'
}
data = urllib.parse.urlencode(payload)
binary_data = data.encode('UTF-8')
req = urllib.request.Request(authentication_url, binary_data)
resp = urllib.request.urlopen(req)
contents = resp.read()

Traceback:

    Traceback (most recent call last):
  File "C:/Python34/loginLinked.py", line 16, in <module>
    resp = urllib.request.urlopen(req)
  File "C:\Python34\lib\urllib\request.py", line 161, in urlopen
    return opener.open(url, data, timeout)
  File "C:\Python34\lib\urllib\request.py", line 469, in open
    response = meth(req, response)
  File "C:\Python34\lib\urllib\request.py", line 579, in http_response
    'http', request, response, code, msg, hdrs)
  File "C:\Python34\lib\urllib\request.py", line 507, in error
    return self._call_chain(*args)
  File "C:\Python34\lib\urllib\request.py", line 441, in _call_chain
    result = func(*args)
  File "C:\Python34\lib\urllib\request.py", line 587, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 403: Forbidden