Error when user is authenticated

## Library version

*What's the installed library version ? Check with `instalooter --version`*:

```
instalooter v2.3.2
```

## Environment

*Describe here your environment, including:*

* *OS*
Debian Buster
* *Python version*
Python 3.6.5
* *`setuptools` version if reporting an issue with installation*
* *non-standard Python implementation if any*
Tried inside a virtualenv

### Reproducible test case

*Are you using the CLI ? If so, include a command that can be used to re-raise the
error, with actual arguments anybody can try:*

(instadownloader) l@stelvio:~/project/instadownloader$ instalooter user test_private -u XXXX -p PPPP --traceback
2019-01-13 11:21:47 stelvio instalooter.cli[1374] CRITICAL
Traceback (most recent call last):
File "/home/l/project/instadowloader/lib/python3.6/site-packages/instalooter/cli/__init__.py", line 151, in main
login(args)
File "/home/l/project/instadowloader/lib/python3.6/site-packages/instalooter/cli/login.py", line 21, in login
InstaLooter._login(username, password)
File "/home/l/project/instadowloader/lib/python3.6/site-packages/instalooter/looters.py", line 143, in _login
token = next(c.value for c in login.cookies if c.name == 'csrftoken')
StopIteration
2019-01-13 11:21:47 stelvio instalooter.cli[1374] WARNING unclosed