URL error

[FinlandBreakfast]

Error
I received the following error when creating a collection of sat objects.

  File "/Users/.../lib/python3.6/urllib/request.py", line 1318, in do_open
    encode_chunked=req.has_header('Transfer-encoding'))
  File "/Users/.../lib/python3.6/http/client.py", line 1239, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/Users/.../lib/python3.6/http/client.py", line 1285, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "/Users/.../lib/python3.6/http/client.py", line 1234, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/Users/.../lib/python3.6/http/client.py", line 1026, in _send_output
    self.send(msg)
  File "/Users/.../lib/python3.6/http/client.py", line 964, in send
    self.connect()
  File "/Users/.../lib/python3.6/http/client.py", line 1392, in connect
    super().connect()
  File "/Users/.../lib/python3.6/http/client.py", line 940, in connect
    self._tunnel()
  File "/Users/.../lib/python3.6/http/client.py", line 919, in _tunnel
    message.strip()))
OSError: Tunnel connection failed: 503 Service Unavailable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "discrete_runner.py", line 122, in <module>
    batch_run.run_all()
  File "/Users/.../lib/python3.6/site-packages/mesa/batchrunner.py", line 130, in run_all
    model_vars, agent_vars = self._run_single_model(param_values, i, kwargscopy)
  File "/Users/.../lib/python3.6/site-packages/mesa/batchrunner.py", line 145, in _run_single_model
    model = self.model_cls(**kwargs)
  File "/Users/.../model.py", line 132, in __init__
    sat = Orbital(tle_object_name) #Fetches TLE from the internet by itself
  File "/Users/.../lib/python3.6/site-packages/pyorbital/orbital.py", line 142, in __init__
    line1=line1, line2=line2)
  File "/Users/.../lib/python3.6/site-packages/pyorbital/tlefile.py", line 147, in read
    return Tle(platform, tle_file=tle_file, line1=line1, line2=line2)
  File "/Users/.../lib/python3.6/site-packages/pyorbital/tlefile.py", line 198, in __init__
    self._read_tle()
  File "/Users/.../lib/python3.6/site-packages/pyorbital/tlefile.py", line 259, in _read_tle
    fid = open_func(url)
  File "/Users/.../lib/python3.6/urllib/request.py", line 223, in urlopen
    return opener.open(url, data, timeout)
  File "/Users/.../lib/python3.6/urllib/request.py", line 526, in open
    response = self._open(req, data)
  File "/Users/.../lib/python3.6/urllib/request.py", line 544, in _open
    '_open', req)
  File "/Users/.../lib/python3.6/urllib/request.py", line 504, in _call_chain
    result = func(*args)
  File "/Users/.../lib/python3.6/urllib/request.py", line 1361, in https_open
    context=self._context, check_hostname=self._check_hostname)
  File "/Users/.../lib/python3.6/urllib/request.py", line 1320, in do_open
    raise URLError(err)
urllib.error.URLError: <urlopen error Tunnel connection failed: 503 Service Unavailable>

Solution
I believe the error is being thrown by too many https calls:

Changing lines 37-42 in pyorbital/tlefile.py to the following seemed to solve the issue.

TLE_URLS = (
            'http://www.celestrak.com/NORAD/elements/iridium.txt',
            'http://www.celestrak.com/NORAD/elements/weather.txt',
            'http://www.celestrak.com/NORAD/elements/resource.txt',
            'http://www.celestrak.com/NORAD/elements/cubesat.txt',
            'http://www.celestrak.com/NORAD/elements/stations.txt',
            'http://www.celestrak.com/NORAD/elements/sarsat.txt',
            'http://www.celestrak.com/NORAD/elements/noaa.txt')