SCons: Python 3 compatibility

[merlinND]

Fixes #82.

Checks needed:

• macOS with SCons @ Python 2
• macOS 10.13 with SCons 3.0.1 @ Python 3.6.5
• Linux with SCons @ Python 2
• Linux with SCons @ Python 3
• Windows with SCons @ Python 2
• Windows with SCons @ Python 3

[bartoszek]

Linux with SCons @ Python 2 works as expected
Linux with SCons @ Python 3 Failed to find qt5
sys={distro:ArchLinux, python:3.8.1, scons:3.1.2, qt5:5.14.1}
scons2.log
scons3.log

Looks like data/scons/qt5.py need a fix.

[gaotongsh]

I created #138 to try to fix data/scons/qt5.py but there are still some bugs.