add cysignals dependency

[keltecc]

fpylll requires cysignals at runtime, but the published wheels do not declare it as a dependency

$ pip install fpylll
$ python3 -c "import fpylll"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import fpylll
  File "/usr/local/lib/python3.14/dist-packages/fpylll/__init__.py", line 3, in <module>
    from .fplll.integer_matrix import IntegerMatrix
  File "src/fpylll/fplll/integer_matrix.pyx", line 1, in init fpylll.fplll.integer_matrix
    # -*- coding: utf-8 -*-
ModuleNotFoundError: No module named 'cysignals'

fix: add cysignals as a dependency in setup.py

[malb]

Ta!