[zope4py3] erp5: Switch python to gpython + spawn runwsgi via it; pre-activate bstr+ustr

Hello @kazuhiko, @jerome.

As asked in https://lab.nexedi.com/nexedi/pygolang/-/merge_requests/21#note_205820 hereby patches show how to adjust stack/erp5/ a bit to run programs via gpython instead of python. All patches except the last one should be good to merge, and mostly to master, because bstr+ustr activation happens only as the last step and all preceeding changes should be making preparatory steps only.

Please see individual patches for details.

Kirill