The version of `cycler` in stack/slapos.cfg is overriding the version
specified in component/matplotlib/buildout.cfg.
This is a problem because the former does not take into account the
Python version, while the later does.
Thus, attempting to compile in Python 2 would give the following error:

```
Getting distribution for 'cycler==0.11.0'.
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
ERROR: Package 'cycler' requires a different Python: 2.7.18 not in '>=3.6'
While:
  Installing matplotlib.
  Base installation request: 'cycler', 'python-dateutil', 'numpy'
  Getting distribution for 'cycler==0.11.0'.
```

This removes `cycler` from `slapos.cfg`, so that it does not fail with
Python 2.