Unknown encoding: cp0

• I am on the latest Hyper.app version
• I have searched the issues of this repo and believe that this is not a duplicate

• OS version and name: Windows 10 1809
• Hyper.app version: 3.0.0
• Link of a Gist with the contents of your .hyper.js: https://gist.github.com/Mossop/b1856e5a9c4a12bd5e423fbca1279fcf
• Relevant information from devtools (CMD+ALT+I on macOS, CTRL+SHIFT+I elsewhere):
• The issue is reproducible in vanilla Hyper.app:

Issue

I just upgraded to Hyper 3, fixed the configuration that it lost and then tried using it. Immediately I found that a python script I need for my work fails to run with an unknown encoding error:

Traceback (most recent call last):
  File "mach", line 86, in <module>
    main(sys.argv[1:])
  File "mach", line 82, in main
    sys.exit(mach.run(args))
  File "c:\mozilla\source\trunk\python/mach\mach\main.py", line 369, in run
    self._print_error_header(argv, sys.stdout)
  File "c:\mozilla\source\trunk\python/mach\mach\main.py", line 581, in _print_error_header
    fh.write('Error running mach:\n\n')
LookupError: unknown encoding: cp0

This is a python script running under an msys bash.

Start by reproducing Hyper 3.0.0 on Windows 10 1809 with the reported .hyper.js and an MSYS Bash session. Read mach and python/mach/mach/main.py around _print_error_header, especially line 581, then verify that the Python script runs without the unknown encoding: cp0 error.