[SPARK-32714][FOLLOW-UP][PYTHON] Address pyspark.install typing errors

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

This PR adds two `type: ignores`, one in `pyspark.install` and one in related tests.

### Why are the changes needed?

To satisfy MyPy type checks. It seems like we originally missed some changes that happened around merge of
31a16fbb40

```
python/pyspark/install.py:30: error: Need type annotation for 'UNSUPPORTED_COMBINATIONS' (hint: "UNSUPPORTED_COMBINATIONS: List[<type>] = ...")  [var-annotated]
python/pyspark/tests/test_install_spark.py:105: error: Cannot find implementation or library stub for module named 'xmlrunner'  [import]
python/pyspark/tests/test_install_spark.py:105: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
```

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

No.

### How was this patch tested?

- Existing tests.
- MyPy tests
    ```
    mypy --show-error-code --no-incremental --config python/mypy.ini python/pyspark
   ```

Closes #29878 from zero323/SPARK-32714-FOLLOW-UP.

Authored-by: zero323 <mszymkiewicz@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

python/pyspark/install.py

@@ -27,7 +27,7 @@ DEFAULT_HADOOP = "hadoop3.2"
 DEFAULT_HIVE = "hive2.3"
 SUPPORTED_HADOOP_VERSIONS = ["hadoop2.7", "hadoop3.2", "without-hadoop"]
 SUPPORTED_HIVE_VERSIONS = ["hive2.3"]
-UNSUPPORTED_COMBINATIONS = [
+UNSUPPORTED_COMBINATIONS = [  # type: ignore
 ]
 
 

python/pyspark/tests/test_install_spark.py

@@ -102,7 +102,7 @@ if __name__ == "__main__":
     from pyspark.tests.test_install_spark import *  # noqa: F401
 
     try:
-        import xmlrunner
+        import xmlrunner  # type: ignore[import]
         testRunner = xmlrunner.XMLTestRunner(output='target/test-reports', verbosity=2)
     except ImportError:
         testRunner = None