Steps to reproduce:
1) Setup local python server with IPv6 support
1. Install Python 3.5
2. Setup Environment variable path so you can use Python in Command line
3. Place 'test.py' script somewhere in your computer
4. Open Command line and navigate this script and launch it with 'python test.py'
5. Open browser and type: http://[::1]:8080 and you should be able to browse directories
2) Open 'repro' project
3) Open 'Untitled' scene and run it
4) In text box type http://[::1]:8080 and press 'Test'

Results: Error: Could not resolve host

Reproduced with: 5.4.0b21, 5.3.5p3, 5.2.4f1