Remove backward compatibility code

[Aranuvir]

Purge all code that provides backward compatibility for Python2 and Blender2.7 since it's a potential source of issues.

[black-punkduck]

have a look on it ... I hope the blender stuff for version < 2.8 and the python 2 lines in MakeHuman plugin are now deleted. I did a few tests ...

I can load a rigify-skeleton also but it looks odd. I am not sure if this support is still needed ... but I will not change it.

@Aranuvir when you think that's okay you can close this issue. I could do it myself I guess ... but a lil' control is not bad I guess :)

[Aranuvir]

Thanks for working at that stuff. I'll take a look at it later.