bug#30121: python-matplotlib broken.

[Fis Trivial]

* Steps to reproduce:

$ python
>>> from matplotlib import pyplot

* Full message:
--8<---------------cut here---------------start------------->8---
Python 3.5.3 (default, Jan  1 1970, 00:00:01)
[GCC 5.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from matplotlib import pyplot
Traceback (most recent call last):
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/backends/qt_compat.py",
 line 176, in <module>
    from PySide import QtCore, QtGui, __version__, __version_info__
ImportError: No module named 'PySide'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/pyplot.py", 
line 115, in <module>
    _backend_mod, new_figure_manager, draw_if_interactive, _show = pylab_setup()
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/backends/__init__.py",
 line 32, in pylab_setup
    globals(),locals(),[backend_name],0)
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/backends/backend_qt5agg.py",
 line 16, in <module>
    from .backend_qt5 import QtCore
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/backends/backend_qt5.py",
 line 26, in <module>
    import matplotlib.backends.qt_editor.figureoptions as figureoptions
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/backends/qt_editor/figureoptions.py",
 line 20, in <module>
    import matplotlib.backends.qt_editor.formlayout as formlayout
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/backends/qt_editor/formlayout.py",
 line 56, in <module>
    from matplotlib.backends.qt_compat import QtGui, QtWidgets, QtCore
  File 
"/home/fis/.guix-profile/lib/python3.5/site-packages/matplotlib/backends/qt_compat.py",
 line 179, in <module>
    "Matplotlib qt-based backends require an external PyQt4, PyQt5,\n"
ImportError: Matplotlib qt-based backends require an external PyQt4, PyQt5,
or PySide package to be installed, but it was not found.
>>>
--8<---------------cut here---------------end--------------->8---

* Platform
Fedora 26 x86_64

* Guix version
guix (GNU Guix) bad12e839c2f7823c45aa0121f7d5c9bb70905b7