Could not get blobedit.py to work on Mint 19/GTK+

[mscheper]

I tried it in my Mint 19/Cinnamon environment. I'm not sure if PyGUI is supposed to work with GTK 3 or 2, so I tried installing both, although maybe I messed something up.

$ sudo apt install libgtk-3-dev python-gtk2
$ cd $git
$ git clone https://github.com/gcewing/PyGUI.git
$ cp -a PyGUI/Demos/BlobEdit $my_projects
$ cd $my_projects/BlobEdit
$ python3 -m venv venv
$ . venv/bin/activate
$ pushd $git/PyGUI
$ python setup.py install
[lots of output; nothing scary-looking; exit status 0]
$ popd
$ python blobedit.py
Traceback (most recent call last):
  File "blobedit.py", line 16, in <module>
    from GUI import Application, ScrollableView, Document, Window, Cursor, rgb
  File "$my_projects/BlobEdit/venv/lib/python3.6/site-packages/GUI/__init__.py", line 54, in <module>
    raise ImportError("Unable to find an implementation of PyGUI for this installation")
ImportError: Unable to find an implementation of PyGUI for this installation

I eventually got past that exception, like this:

$ pip install vext
$ pip install vext.gi
$ pip install PyGObjectg

But then I got stuck with:

$my_projects/BlobEdit/venv/lib/python3.6/site-packages/GUI/__init__.py:49: PyGIWarning: Gtk was imported without specifying a version first. Use gi.require_version('Gtk', '3.0') before import to ensure that the right version gets loaded.
  __import__(_testmod)
Traceback (most recent call last):
  File "blobedit.py", line 16, in <module>
    from GUI import Application, ScrollableView, Document, Window, Cursor, rgb
  File "$my_projects/BlobEdit/venv/lib/python3.6/site-packages/GUI/__init__.py", line 74, in <module>
    from GUI.Colors import rgb
  File "$my_projects/BlobEdit/venv/lib/python3.6/site-packages/GUI/Colors.py", line 5, in <module>
    from gtk import Style
ImportError: cannot import name 'Style'

I tried it in a new venv with --system-site-packages, but this didn't help. And of course, I don't want to run PyGUI's setup.py without a venv.

Advice would be appreciated, and I suggest you add solutions for problems like this in README.md, especially until #1 is resolved.