Update developer dependencies

[qstokkink]

Related to #3265. I had to do the following to get Tribler to run from source on Ubuntu 18 (using pip as much as possible):

# Install python
sudo apt install python2.7 python-minimal python-pip

# Install py-ipv8 python dependencies
cd Tribler/pyipv8
pip install --upgrade -r requirements.txt

# Install sytem dependencies
sudo apt install ffmpeg libssl-dev libx11-6 vlc

# Install Tribler python dependencies
sudo apt install python-apsw python-cherrypy3 python-keyring python-keyrings.alt python-libtorrent python-m2crypto python-meliae python-pil python-pyqt5 python-pyqt5.qtsvg python-scipy
pip install bitcoinlib chardet configobj decorator dnspython ecdsa feedparser jsonrpclib leveldb matplotlib netifaces networkx pbkdf2 pony protobuf psutil pyaes pyasn1 pysocks requests

pip install service_identity segfaults for me so I have to do:

git clone --recursive https://github.com/pyca/service_identity
cd service_identity
sudo python setup.py install

[ichorid]

Still broken?

[devos50]

I'm pretty confident that this is sufficiently addressed, given our recent changes to the developer documentation. This issue did also concern Python 2.

[qstokkink]

Yes, this was fixed by #4454 (but that PR didn't use the GitHub fixes auto-close apparently).