Bug#1122831: python3-scipy dependency is missing

[VA]

Package: graphs
Version: 1.8.5-1

The python3-scipy dependency is missing, else:

```
Traceback (most recent call last):
   File "/usr/bin/graphs", line 68, in <module>
     from graphs.application import PythonApplication
   File "/usr/lib/python3/dist-packages/graphs/application.py", line 8, 
in <module>
     from graphs import (
     ...<7 lines>...
     )
   File "/usr/lib/python3/dist-packages/graphs/actions.py", line 9, in 
<module>
     from graphs import file_io, project, ui, utilities
   File "/usr/lib/python3/dist-packages/graphs/ui.py", line 9, in <module>
     from graphs.item_box import ItemBox
   File "/usr/lib/python3/dist-packages/graphs/item_box.py", line 7, in 
<module>
     from graphs.curve_fitting import CurveFittingDialog
   File "/usr/lib/python3/dist-packages/graphs/curve_fitting.py", line 
14, in <module>
     from scipy.optimize import _minpack, curve_fit
ModuleNotFoundError: No module named 'scipy'
```