[SPARK-54065][PYTHON][TESTS] Enable test_in_memory_data_source in Python 3.14

### What changes were proposed in this pull request?

This PR aims to enable `test_in_memory_data_source` in Python 3.14.

### Why are the changes needed?

After upgrading to the latest `cloudpickle` 3.1.2, I verified that the test passed on Python 3.14.0.
- #52964

```
$ python/run-tests --parallelism 1 --testnames pyspark.sql.tests.test_python_datasource --python-executables python3
Running PySpark tests. Output is in /Users/dongjoon/APACHE/spark-merge/python/unit-tests.log
Will test against the following Python executables: ['python3']
Will test the following Python tests: ['pyspark.sql.tests.test_python_datasource']
python3 python_implementation is CPython
python3 version is: Python 3.14.0
Starting test(python3): pyspark.sql.tests.test_python_datasource (temp output: /Users/dongjoon/APACHE/spark-merge/python/target/e771105c-5304-4f4f-bea6-b8d896b1b250/python3__pyspark.sql.tests.test_python_datasource__2wyrv5n4.log)
Finished test(python3): pyspark.sql.tests.test_python_datasource (29s)
Tests passed in 29 seconds
```

### Does this PR introduce _any_ user-facing change?

No. This is a test change.

### How was this patch tested?

Pass the CIs and manual tests.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #52967 from dongjoon-hyun/SPARK-54065.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

python/pyspark/sql/tests/test_python_datasource.py

@@ -258,7 +258,6 @@ def test_read_with_invalid_return_row_type(self):
         with self.assertRaisesRegex(PythonException, "DATA_SOURCE_INVALID_RETURN_TYPE"):
             df.collect()
 
-    @unittest.skipIf(sys.version_info > (3, 13), "SPARK-54065")
     def test_in_memory_data_source(self):
         class InMemDataSourceReader(DataSourceReader):
             DEFAULT_NUM_PARTITIONS: int = 3