Requests inexplicably aborts connection to https://dictionary.cambridge.org

I also tested the same url with `urllib.request.urlopen` and it works fine.

## Expected Result

To get a connection to https://dictionary.cambridge.org.

## Actual Result

Requests fails with the error:
```
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py", line 600, in urlopen
    chunked=chunked)
  File "/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py", line 384, in _make_request
    six.raise_from(e, None)
  File "<string>", line 2, in raise_from
  File "/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py", line 380, in _make_request
    httplib_response = conn.getresponse()
  File "/usr/lib/python3.6/http/client.py", line 1331, in getresponse
    response.begin()
  File "/usr/lib/python3.6/http/client.py", line 297, in begin
    version, status, reason = self._read_status()
  File "/usr/lib/python3.6/http/client.py", line 266, in _read_status
    raise RemoteDisconnected("Remote end closed connection without"
http.client.RemoteDisconnected: Remote end closed connection without response

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/requests/adapters.py", line 449, in send
    chunked=chunked
  File "/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py", line 638, in urlopen
    _stacktrace=sys.exc_info()[2])
  File "/usr/local/lib/python3.6/dist-packages/urllib3/util/retry.py", line 367, in increment
    raise six.reraise(type(error), error, _stacktrace)
  File "/usr/local/lib/python3.6/dist-packages/urllib3/packages/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py", line 600, in urlopen
    chunked=chunked)
  File "/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py", line 384, in _make_request
    six.raise_from(e, None)
  File "<string>", line 2, in raise_from
  File "/usr/local/lib/python3.6/dist-packages/urllib3/connectionpool.py", line 380, in _make_request
    httplib_response = conn.getresponse()
  File "/usr/lib/python3.6/http/client.py", line 1331, in getresponse
    response.begin()
  File "/usr/lib/python3.6/http/client.py", line 297, in begin
    version, status, reason = self._read_status()
  File "/usr/lib/python3.6/http/client.py", line 266, in _read_status
    raise RemoteDisconnected("Remote end closed connection without"
urllib3.exceptions.ProtocolError: ('Connection aborted.', RemoteDisconnected('Remote end closed connection without response',))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/requests/api.py", line 76, in get
    return request('get', url, params=params, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/requests/api.py", line 61, in request
    return session.request(method=method, url=url, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/requests/sessions.py", line 530, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/local/lib/python3.6/dist-packages/requests/sessions.py", line 643, in send
    r = adapter.send(request, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/requests/adapters.py", line 453, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', RemoteDisconnected('Remote end closed connection without response',))
```

## Reproduction Steps

```python
import requests
requests.get('https://dictionary.cambridge.org')
```

## System Information

    $ python -m requests.help

```
{
  "chardet": {
    "version": "3.0.4"
  },
  "cryptography": {
    "version": "2.1.4"
  },
  "idna": {
    "version": "2.6"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.6.7"
  },
  "platform": {
    "release": "4.15.0-64-generic",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "1010007f",
    "version": "17.5.0"
  },
  "requests": {
    "version": "2.22.0"
  },
  "system_ssl": {
    "version": "1010100f"
  },
  "urllib3": {
    "version": "1.24.1"
  },
  "using_pyopenssl": true
}
```

I don't understand much of what is happening at the lower level and I'm sure that is where the error is at (connection headers, ect.). I also came across this error which, if you could explain, I'd be much obliged:
```
urllib.request.urlopen('https://www.enotes.com')
```
Gives the error:
```
Traceback (most recent call last):
  File "references_to_urls.py", line 60, in <module>
    html = urllib.request.urlopen(url).read().decode('utf-8', errors='ignore')
  File "/usr/lib/python3.6/urllib/request.py", line 223, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python3.6/urllib/request.py", line 532, in open
    response = meth(req, response)
  File "/usr/lib/python3.6/urllib/request.py", line 642, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.6/urllib/request.py", line 570, in error
    return self._call_chain(*args)
  File "/usr/lib/python3.6/urllib/request.py", line 504, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.6/urllib/request.py", line 650, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 403: Forbidden
```
However the same link works with requests!

Using a user-agent (`requests.get('https://dictionary.cambridge.org', headers={'User-agent': 'Mozilla/5.0'})`) fixes the problem for requests. I wonder if urllib automatically uses a user-agent?

Yes, urllib automatically adds a user agent. Cambridge has chosen to block scraping with Requests by aborting connections with that user-agent.