`make generate` fails on xpybutil and system_hotkey

[Roming22]

The error is thrown by https://github.com/flatpak/flatpak-builder-tools/blob/master/pip/flatpak-pip-generator:

Traceback (most recent call last):
  File "build-aux/flatpak-pip-generator", line 227, in <module>
    url = get_tar_package_url_pypi(name, version)
  File "build-aux/flatpak-pip-generator", line 64, in get_tar_package_url_pypi
    raise Exception(err)
Exception: Failed to get xpybutil-0.0.6 source from https://pypi.org/pypi/xpybutil/0.0.6/json

Reverting to an older version of flatpak-pip-generator solves the issue (I tried 3b460d29d6c58eabdccce2595faa2d512fa0259d since it was likely the version last used by the main developer, as the enxt commit remove any.whl support).

Edit: Changed latest know working version