Fix Python 3.14t free-threaded build: use Py_REFCNT() instead of ob_refcnt

- Declare Py_REFCNT() as external C function from Python.h
- Replace direct ob_refcnt access with Py_REFCNT() macro (2 occurrences)
- Add NULL checks before calling Py_REFCNT() to prevent null pointer dereference
- This fixes compilation errors in Python 3.14t (free-threaded/nogil) build
  where ob_refcnt is not directly accessible in PyObject structure
- All 66 tests pass

Author: cxzhong

src/cysignals/signals.pyx

@@ -48,6 +48,10 @@ cdef extern from *:
     """
     pass
 
+# Declare Py_REFCNT for Python 3.14t free-threaded build compatibility
+cdef extern from "Python.h":
+    Py_ssize_t Py_REFCNT(PyObject *ob)
+
 cdef extern from "implementation.c":
     cysigs_t cysigs
     int _set_debug_level(int) nogil
@@ -361,7 +365,7 @@ cdef void verify_exc_value() noexcept:
     Check that ``cysigs.exc_value`` is still the exception being raised.
     Clear ``cysigs.exc_value`` if not.
     """
-    if cysigs.exc_value.ob_refcnt == 1:
+    if cysigs.exc_value != NULL and Py_REFCNT(cysigs.exc_value) == 1:
         # No other references => exception is certainly gone
         Py_CLEAR(cysigs.exc_value)
         return
@@ -409,5 +413,5 @@ cdef void verify_exc_value() noexcept:
     # Make sure we still have cysigs.exc_value at all; if this function was
     # called again during garbage collection it might have already been set
     # to NULL; see https://github.com/sagemath/cysignals/issues/126
-    if cysigs.exc_value != NULL and cysigs.exc_value.ob_refcnt == 1:
+    if cysigs.exc_value != NULL and Py_REFCNT(cysigs.exc_value) == 1:
         Py_CLEAR(cysigs.exc_value)