[SPARK-23698][PYTHON][FOLLOWUP] Resolve undefiend names in setup.py

## What changes were proposed in this pull request?

`__version__` in `setup.py` is currently being dynamically read by `exec`; so the linter complains. Better just switch it off for this line for now.

**Before:**

```bash
$ python -m flake8 . --count --select=E9,F82 --show-source --statistics
./setup.py:37:11: F821 undefined name '__version__'
VERSION = __version__
          ^
1     F821 undefined name '__version__'
1
```

**After:**

```bash
$ python -m flake8 . --count --select=E9,F82 --show-source --statistics
0
```

## How was this patch tested?

Manually tested.

Closes apache#22235 from HyukjinKwon/SPARK-23698.

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

Author: HyukjinKwon

python/setup.py

@@ -34,7 +34,7 @@
     print("Failed to load PySpark version file for packaging. You must be in Spark's python dir.",
           file=sys.stderr)
     sys.exit(-1)
-VERSION = __version__
+VERSION = __version__  # noqa
 # A temporary path so we can access above the Python project root and fetch scripts and jars we need
 TEMP_PATH = "deps"
 SPARK_HOME = os.path.abspath("../")