Hi everybody, a little update about this patch,

I just found out that it doesn't actually fix the problem: the 
requirement for prompt 2 is enforced also at runtime.

The output of guix package _without_ my patch is:


$ guix package -p /tmp/tmp-profile -i jupyter
The following package will be installed:
    jupyter 1.0.0

guix package: error: profile contains conflicting entries for python-ipython
guix package: error:   first entry: python-ipython@7.9.0 
/gnu/store/z0yarar134q6sziqncjdlfg2h2w5b2yh-python-ipython-7.9.0
guix package: error:    ... propagated from python-ipywidgets@7.5.1
guix package: error:    ... propagated from jupyter@1.0.0
guix package: error:   second entry: python-ipython@7.9.0 
/gnu/store/avmg5jd7kccchxadsd9c7zq0p3shlk3w-python-ipython-7.9.0
guix package: error:    ... propagated from python-qtconsole@4.4.3
guix package: error:    ... propagated from jupyter@1.0.0
hint: You cannot have two different versions or variants of `jupyter' in 
the same profile.


With my patch applied it is possible to install jupyter without 
conflicts but when started it crashes asking for prompt-toolkit at 
version 2. The conflict seems to be caused because:

- 
/gnu/store/avmg5jd7kccchxadsd9c7zq0p3shlk3w-python-ipython-7.9.0depends 
on python-prompt-toolkit@3.0.5

- 
/gnu/store/z0yarar134q6sziqncjdlfg2h2w5b2yh-python-ipython-7.9.0depends 
on python-prompt-toolkit@2.0.7

Please ignore the previous patch, I apologize for the noise. I'm opening 
an actual bug report and I'll start working on a fix ASAP.


Giacomo