Reports of error in `freeze_support` with Windows installer

[RalfG]

With v3.0.0b1 of the Windows Installer:

  File "ms2rescore\gui\__main__.py", line 15, in <module>
    main()
  File "ms2rescore\gui\__main__.py", line 10, in main
    multiprocessing.freeze_support()
  File "Lib\site-packages\PyInstaller\hooks\rthooks\pyi_rth_multiprocessing.py", line 47, in _freeze_support
ValueError: not enough values to unpack (expected 2, got 1)

Potentially related to https://github.com/pyinstaller/pyinstaller/issues/7482