Installation instructions for python3

[NormB]

I have installed the opensips-cli package from source using these instructions:
docs/INSTALLATION.md.

The installation failed because the use of "python setup.py install" has been deprecated:

See this document for information about replacement commands:
https://packaging.python.org/en/latest/discussions/setup-py-deprecated/

I found that on a debian:bookworm image the following command was required if a virtual environment for the installation of python modules was not used:

python3 -m pip install --break-system-packages clean .

[jwarnier]

I believe #155 might fix this for you.