pip - AttributeError: module 'subprocess' has no attribute 'mswindows'

[gliberal]

On ArchLinux, when doing pip --user install WebUI, the installation fails due to PySide version :

Collecting pyside==1.2.1 (from WebUI)
  Using cached https://files.pythonhosted.org/packages/f1/32/63872fdc1d9e0de5aa0b05b9d1a17d868851c619cdc6998dd9d853556f1b/PySide-1.2.1.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-f_q789bo/pyside/setup.py", line 82, in <module>
        from utils import rmtree
      File "/tmp/pip-install-f_q789bo/pyside/utils.py", line 10, in <module>
        import popenasync
      File "/tmp/pip-install-f_q789bo/pyside/popenasync.py", line 26, in <module>
        if subprocess.mswindows:
    AttributeError: module 'subprocess' has no attribute 'mswindows'

I have the same error in Python 3.6 & 3.7. I know I could use the pip install with your git repo directly but I prefer to avoid that in my requirements.txt.

Thanks for your work and have a nice day :)