Make pyrevit module compatible with python 3

[eirannejad]

[sanzoghenzo]

Given that pythonnet v3 milestone due date has not been disclosed yet, would you accept a PR to fix the current incompatibility?

I'm not sure if I can solve everything, but I could try it.

But first, a quick question: how can I load a git clone of pyrevit(lib) in revit?

Forgive me if there's already an answer somewhere, but I can't find any info. The first place I tried was CONTRIBUTING.md, but it's empty..

[emilaz]

The pythonnet 3.0 version should be released any day now, with v3.0.0-rc4 already out (https://github.com/pythonnet/pythonnet/releases). Maybe we can start switching to pythonnet v3, as the remaining changes to v3 will be miniscule? The faster we move away from the badly maintained security risk that is IronPython2.7 the better :)