[SPARK-38297][PYTHON] Explicitly cast the return value at DataFrame.t…

…o_numpy in POS

MyPy build currently fails as below:

```
starting mypy annotations test...
annotations failed mypy checks:
python/pyspark/pandas/generic.py:585: error: Incompatible return value type (got "Union[ndarray[Any, Any], ExtensionArray]", expected "ndarray[Any, Any]")  [return-value]
Found 1 error in 1 file (checked 324 source files)
1
```

https://github.com/apache/spark/runs/5298261168?check_suite_focus=true

I tried to reproduce in my local by matching NumPy and MyPy versions but failed. So I decided to work around the problem first by explicitly casting to make MyPy happy.

To make the build pass.

No, dev-only.

CI in this PR should verify if it's fixed.

Closes #35617 from HyukjinKwon/SPARK-38297.

Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
(cherry picked from commit b46b74c)
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/pyspark/pandas/generic.py

@@ -573,7 +573,7 @@ def to_numpy(self) -> np.ndarray:
         >>> ps.Series(['a', 'b', 'a']).to_numpy()
         array(['a', 'b', 'a'], dtype=object)
         """
-        return self.to_pandas().values
+        return cast(np.ndarray, self.to_pandas().values)
 
     @property
     def values(self) -> np.ndarray: