[SPARK-39030][PYTHON] Rename sum to avoid shading the builtin Python …

…function

### What changes were proposed in this pull request?
Rename sum to something else.

### Why are the changes needed?
Sum is a build in function in python. [SUM() at python docs](https://docs.python.org/3/library/functions.html#sum)

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Use existing tests.

Closes apache#36364 from bjornjorgensen/rename-sum.

Authored-by: bjornjorgensen <bjornjorgensen@gmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
(cherry picked from commit 3821d80)
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/pyspark/pandas/base.py

@@ -1326,8 +1326,8 @@ def value_counts(
                 sdf = sdf.orderBy(F.col("count").desc())
 
         if normalize:
-            sum = sdf_dropna.count()
-            sdf = sdf.withColumn("count", F.col("count") / SF.lit(sum))
+            drop_sum = sdf_dropna.count()
+            sdf = sdf.withColumn("count", F.col("count") / SF.lit(drop_sum))
 
         internal = InternalFrame(
             spark_frame=sdf,