New release version number

[eoyilmaz]

Now that DisplayCAL is working with Python 3.8+ we need to have a new version number. I'm tempted to call it 4.0.0 (also getting rid of the 4 digit version numbering, which doesn't follow semantic versioning: https://semver.org/ ).

But, increasing the major version from 3 to 4 requires a huge change in the way the system is interpreting data, that makes it incompatible with the 3 series. But, DisplayCAL is compatible with data created with Python 2.x version. So, this makes me think that we need to call it 3.9.0 and not 4.0.0 and reserve 4.0.0 for huge changes like implementing Qt, changing the inner workings of DisplayCAL to make it more Pythonic etc.

[eoyilmaz]

Okay it is officially 3.9.0 now.