Fix missing skip_if_rust_bson import in test_custom_types.py

The decorator was being used but not imported, causing NameError
during test collection.

Author: aclark4life

test/test_custom_types.py

@@ -196,14 +196,12 @@ def test_decode_file_iter(self):
         fileobj.close()
 
 
-@skip_if_rust_bson("Decimal/Decimal128 custom type codecs not yet implemented")
 class TestCustomPythonBSONTypeToBSONMonolithicCodec(CustomBSONTypeTests, unittest.TestCase):
     @classmethod
     def setUpClass(cls):
         cls.codecopts = DECIMAL_CODECOPTS
 
 
-@skip_if_rust_bson("Decimal/Decimal128 custom type codecs not yet implemented")
 class TestCustomPythonBSONTypeToBSONMultiplexedCodec(CustomBSONTypeTests, unittest.TestCase):
     @classmethod
     def setUpClass(cls):