[taurus5] Remove python2 and Qt4 support
The main step towards taurus 5 release:
- remove support for python 2 (closes #1041 (closed))
- remove support for Qt4 (closes #1196 (closed) )
The above changes include cleaning the source code of "compatibility hacks" for py2 and Qt4
TO-DO: for the taurus 5 release I will also submit a separate MR on top of this one to make taurus flake8-clean