change to pip install for bugfix
BUGFIX
Our CI fails because the astropy version we are using (installed by apt-get install python3-astropy)
is still using the deprecated function numpy.asscalar
.
This bug is fixed in a newer astropy version that one gets by installing via pip.
@mtr @parras what do you think?
Alternatively we could go for an older version of numpy. But I prefer this approach.