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urllib.robotparser.RobotFileParser always uses the default Python user agent. This agent is now blacklisted by many sites, and it's not possible to read the robots.txt file at all.

Suggest adding a user_agent optional parameter, as shown here:
 def __init__(self, url='', user_agent=None):
 urllib.robotparser.RobotFileParser.__init__(self, url) # init parent
 self.user_agent = user_agent # save user agent
 
 def read(self):
 """
 Reads the robots.txt URL and feeds it to the parser.
 Overrides parent read function.
 """
 try:
 req = urllib.request.Request( # request with user agent specified
 self.url, 
 data=None)
 if self.user_agent is not None : # if overriding user agent
 req.add_header("User-Agent", self.user_agent)
 f = urllib.request.urlopen(req) # open connection
 except urllib.error.HTTPError as err:
 if err.code in (401, 403):
 self.disallow_all = True
 elif err.code >= 400 and err.code < 500:
 self.allow_all = True
 else:
 raw = f.read()
 self.parse(raw.decode("utf-8").splitlines())

(That's from a subclass I wrote. As a change to RobotFileParser, __init__ should start like this.)
 def __init__(self, url='', user_agent=None):
 self.user_agent = user_agent # save user agent
 ...

Hi, John. This issue of robotparser has been reported in #15851. I'll close this as duplicate and you can discuss in that thread.