[SPARK-34630][PYTHON] Add typehint for pyspark.__version__

### What changes were proposed in this pull request?
This PR adds the typehint of pyspark.__version__, which was mentioned in [SPARK-34630](https://issues.apache.org/jira/browse/SPARK-34630).

### Why are the changes needed?
There were some short discussion happened in https://github.com/apache/spark/pull/31823#discussion_r593830911 .

After further deep investigation on [1][2], we can see the `pyspark.__version__` is added by [setup.py](c06758834e/python/setup.py (L201)), it makes `__version__` embedded into pyspark module, that means the `__init__.pyi` is the right place to add the typehint for `__version__`.

So, this patch adds the type hint `__version__` in pyspark/__init__.pyi.

[1] [PEP-396 Module Version Numbers](https://www.python.org/dev/peps/pep-0396/)
[2] https://packaging.python.org/guides/single-sourcing-package-version/
### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
1. Disable the ignore_error on
ee7bf7d962/python/mypy.ini (L132)

2. Run mypy:
- Before fix
```shell
(venv) ➜  spark git:(SPARK-34629) ✗ mypy --config-file python/mypy.ini python/pyspark | grep version
python/pyspark/pandas/spark/accessors.py:884: error: Module has no attribute "__version__"
```

- After fix
```shell
(venv) ➜  spark git:(SPARK-34629) ✗ mypy --config-file python/mypy.ini python/pyspark | grep version
```
no output

Closes #32110 from Yikun/SPARK-34629.

Authored-by: Yikun Jiang <yikunkero@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

python/pyspark/__init__.pyi

@@ -71,3 +71,5 @@ def copy_func(
     doc: Optional[str] = ...,
 ) -> F: ...
 def keyword_only(func: F) -> F: ...
+
+__version__: str