Apart from the options already given in other answers, there’s a current more active, recent and open-source project called pygubu.
This is the first description by the author taken from the github repository:
Pygubu is a RAD tool to enable quick & easy development of user
interfaces for the python tkinter module.The user interfaces designed are saved as XML, and by using the pygubu
builder these can be loaded by applications dynamically as needed.
Pygubu is inspired by Glade.
Pygubu hello world program is an introductory video explaining how to create a first project using Pygubu.
The following in an image of interface of the last version of pygubu designer on a OS X Yosemite 10.10.2:
I would definitely give it a try, and contribute to its development.