Fix CFFI typing

[wrdxwrdxwrdx]

src/pysatl_core/sampling/unuran/bindings/_cffi_build.py, line 21:

from cffi import FFI  # type: ignore[import-untyped]

Problem

There are two distinct issues here that compound each other.

1. The suppression comment itself is now an error

Running mypy in strict mode (as configured in pyproject.toml) produces:

src/pysatl_core/sampling/unuran/bindings/_cffi_build.py:21:
    error: Unused "type: ignore" comment  [unused-ignore]

Under the current mypy version, importing cffi does not raise [import-untyped], so the
suppression comment is dead code — but the [unused-ignore] error it produces is a real mypy
error that breaks strict-mode checking.

2. cffi ships no type information at all

Checking the installed package:

py.typed present : False
.pyi stub files  : []

cffi has neither a py.typed marker nor bundled .pyi stubs. Because mypy silently treats the
import as Any-typed rather than raising [import-untyped], the suppression goes undetected —
but the underlying lack of types is real.

Every object that flows through cffi is typed as Any:

Object	Declared type	Runtime type
ffi (FFI instance)	Any	cffi.api.FFI
lib (compiled extension)	Any	_cffi_backend.Lib
CFFI callbacks	Any	_cffi_backend.__CDataOwnGC
unuran_distr, unuran_gen, unuran_par	Any	CFFI pointers

All call sites in callbacks.py, dgt.py, initialization.py, and unuran_sampler.py annotate
these as Any precisely because cffi gives no types. This means:

• Wrong argument counts or types in lib.unur_*() calls are invisible to the type checker.
• Incorrect CFFI callback signatures (e.g. passing "double(double)" where "double(int, ...)" is
  expected) are not caught at definition time.
• Returning a wrong type from a callback goes unnoticed.

Minimal reproduction

$ poetry run mypy src/pysatl_core/sampling/unuran/bindings/_cffi_build.py
src/pysatl_core/sampling/unuran/bindings/_cffi_build.py:21:
    error: Unused "type: ignore" comment  [unused-ignore]