Using unittest.SkipTest exception for skipping tests.

`absltest.skip` is producing a warning with Python 3.12

ml_dtypes/tests/custom_float_test.py

@@ -794,7 +794,7 @@ def testArange(self, float_type):
  np.arange(1, 100, dtype=float_type),
  )
  if float_type == float8_e8m0fnu:
- return absltest.skip("Skip negative ranges for E8M0.")
+ raise self.skipTest("Skip negative ranges for E8M0.")
 
  np.testing.assert_equal(
  np.arange(-8, 8, 1, dtype=np.float32).astype(float_type),
@@ -975,7 +975,7 @@ def testFrexp(self, float_type):
 
  def testCopySign(self, float_type):
  if not dtype_is_signed(float_type):
- return absltest.skip("Skip copy sign test for unsigned floating formats.")
+ raise self.skipTest("Skip copy sign test for unsigned floating formats.")
 
  for bits in list(range(1, 128)):
  with self.subTest(bits):