Use Py_REFCNT over ob_refcnt for reference count checks

Fixes

```
FAILED: [code=1] src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/meson-generated_src_cysignals_signals.pyx.c.o
cc -Isrc/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p -Isrc/cysignals -I../src/cysignals -Isrc -I../src -I/opt/_internal/cpython-3.14.0rc2-nogil/include/python3.14t -fvisibility=hidden -fdiagnostics-color=always -DNDEBUG -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -O3 -DCYTHON_CLINE_IN_TRACEBACK=0 -U_FORTIFY_SOURCE -fPIC -pthread -MD -MQ src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/meson-generated_src_cysignals_signals.pyx.c.o -MF src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/meson-generated_src_cysignals_signals.pyx.c.o.d -o src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/meson-generated_src_cysignals_signals.pyx.c.o -c src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/src/cysignals/signals.pyx.c
src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/src/cysignals/signals.pyx.c: In function ‘__pyx_f_9cysignals_7signals_verify_exc_value’:
src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/src/cysignals/signals.pyx.c:4699:32: error: ‘PyObject’ {aka ‘struct _object’} has no member named ‘ob_refcnt’
4699 | __pyx_t_1 = (cysigs.exc_value->ob_refcnt == 1);
| ^~
src/cysignals/signals.cpython-314t-x86_64-linux-gnu.so.p/src/cysignals/signals.pyx.c:4996:33: error: ‘PyObject’ {aka ‘struct _object’} has no member named ‘ob_refcnt’
4996 | __pyx_t_10 = (cysigs.exc_value->ob_refcnt == 1);
| ^~
```

Author: tobiasdiez

src/cysignals/signals.pyx

@@ -25,7 +25,7 @@ See ``tests.pyx`` for extensive tests.
 
 from libc.signal cimport *
 from libc.stdio cimport freopen, stdin
-from cpython.ref cimport Py_XINCREF, Py_CLEAR
+from cpython.ref cimport Py_XINCREF, Py_CLEAR, _Py_REFCNT
 from cpython.exc cimport (PyErr_Occurred, PyErr_NormalizeException,
         PyErr_Fetch, PyErr_Restore)
 from cpython.version cimport PY_MAJOR_VERSION
@@ -360,7 +360,7 @@ 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
@@ -408,5 +408,5 @@ 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)