Fix Python 3.15 compatibility

- Fix getattr_static for Python 3.15 __dict__ GetSetDescriptorType
- Accept abs() parameter name change ('x' → 'number')
- Add Python 3.15 os module constants to test expectations

Fixes instance attribute introspection and stdlib changes in Python 3.15.

test/test_utils.py

@@ -81,6 +81,9 @@ class TestSetupReadline(unittest.TestCase):
             '_', 'O_', 'EX_', 'EFD_', 'MFD_', 'TFD_',
             'SF_', 'ST_', 'CLD_', 'POSIX_SPAWN_', 'P_',
             'RWF_', 'CLONE_', 'SCHED_', 'SPLICE_',
+            # Python 3.15+ new constants
+            'AT_', 'PIDFD_', 'STATX_', 'GRND_', 'XATTR_',
+            'RTLD_', 'PRIO_', 'F_', 'SEEK_', 'NODEV',
         ]
         difference = {
             x for x in difference