python-cypari2 should depend on python-cysignals-*

[Riad S. Wahby]

Hello folks,

In unstable and testing, python-cypari2 doesn't depend on any of
the cysignals packages. This means that installing python-cypari2 by
itself results in a broken package:

    Python 2.7.15 (default, Jul 28 2018, 11:29:29) 
    [GCC 8.1.0] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import cypari2
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python2.7/dist-packages/cypari2/__init__.py", line 1, in <module>
        from .pari_instance import Pari
      File "cypari2/gen.pxd", line 5, in init cypari2.pari_instance
      File "cypari2/gen.pyx", line 1, in init cypari2.gen
    ImportError: No module named cysignals.signals

Installing either python-cysignals-bare or python-cysignals-pari
addresses this error (but perhaps one or the other of these is a
better choice---I'm not sure).

So it appears to me that the python-cypari2 package is missing a
dependency on one of these packages.

Thanks for all your hard work!

-=rsw