Installing qibo breaks installation from source

[stavros11]

Describe the bug

I noticed some strange behavior while trying to test qiboteam/qibotf#17 in my laptop. It seems that installing qibo using python setup.py develop breaks some package used by pip install / python setup.py making it impossible to install other libraries after.

To Reproduce

1. Create a clean python environment, for example using python -m virtualenv test-env
2. Install qibo from source using python setup.py develop. This ends with the following error:

error: pyparsing 3.0.4 is installed but pyparsing<3,>=2.0.2 is required by {'packaging'}

but qibo is still successfully installed and usable.
3. Try to install some other library, eg. qibojit, using python setup.py develop. This fails with the following error:

pkg_resources.ContextualVersionConflict: (pyparsing 3.0.4 (/home/stavros/PythonEnvs/test2-env/lib/python3.8/site-packages/pyparsing-3.0.4-py3.8.egg), Requirement.parse('pyparsing<3,>=2.0.2'), {'packaging'})

and qibojit is not usable. It is still possible to install libraries from pypi using pip, but not from source using pip install ..

It is the first time I notice something like this.

[mlazzarin]

It looks like a conflict between sub-dependencies. Take a look at pypa/packaging#471 and pypa/packaging#481.

[mlazzarin]

As a temporary fix, you can remove packaging==21.x and install packaging==21.0.