Vispy dependency breaks the client on Ubuntu 18.04

[ataffanel]

The introduction of the vispy dependency in ticker #449 breaks the client on Ubuntu 18. On Linux we tend to use the pyqt version from the distribution, python3-pyt5.

I can see two different way forward to solve this problem

• Use a vispy version that is compatible with the Ubuntu18 qt version
• Install PyQt using pip on Linux (this would be idea but we need to make sure this is not causing problems with already installed Qt versions).

[ataffanel]

Using the Ubuntu version of vispy does not work as it is too old. Using PyQT from pip does work so this is what we should use. Using venv or installing only as user (ie. never doing sudo pip install ...) seems to avoid compatibility problems.

I can see the following steps to solve this issue:

• Make pyqt5 a default dependency when installing the client with pip install
• Update README to clean up all Linux/Python instructions. It should only document pip install procedure, without sudo and with a chapter about venv.
• Update snap build instructions