When trying to install Pyobjus: NameError: name 'build_ext' is not defined

[HeRo002]

I am trying to install Pyobjus in a Python venv on an Ubuntu 20.04 linux.
In Termintal I activate the venv, then I go to the directory where I unpacked 'pyobjus-master'. There I have tried different install methods, but they all result in variations of this:

Traceback (most recent call last):
  File "setup.py", line 35, in <module>
    class PyObjusBuildExt(build_ext, object):
NameError: name 'build_ext' is not defined

What am I doing wrong?

PS: The venv has a large amount of packages installed, and all up-to-date, including:
setuptools 54.1.2
cython 0.29.22
(Someone mentioned them in another issue. That's why I mention them here.)

PPS: My real goal is to be able to export/share a file from a Kivy app to other apps on iOS/iPhone. I am willing to pay someone for making that possible!

Please help!

[HeRo002]

Well... I looked at 'setup.py'. It seems it can only be installed on Mac OS X and iOS... I guess that makes sense. :-)