[SPARK-32447][CORE] Use python3 by default in pyspark and find-spark-…

…home scripts

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

This PR aims to use `python3` instead of `python` inside `bin/pyspark`, `bin/find-spark-home` and `bin/find-spark-home.cmd` script.
```
$ git diff master --stat
 bin/find-spark-home     | 4 ++--
 bin/find-spark-home.cmd | 4 ++--
 bin/pyspark             | 4 ++--
```

### Why are the changes needed?

According to [PEP 394](https://www.python.org/dev/peps/pep-0394/), we have four different cases for `python` while `python3` will be there always.
```
- Distributors may choose to set the behavior of the python command as follows:
      python2,
      python3,
      not provide python command,
      allow python to be configurable by an end user or a system administrator.
```

Moreover, these scripts already depend on `find_spark_home.py` which is using `#!/usr/bin/env python3`.
```
FIND_SPARK_HOME_PYTHON_SCRIPT="$(cd "$(dirname "$0")"; pwd)/find_spark_home.py"
```

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

No. Apache Spark 3.1 already drops Python 2.7 via SPARK-32138 .

### How was this patch tested?

Pass the Jenkins or GitHub Action.

Closes apache#29246 from dongjoon-hyun/SPARK-FIND-SPARK-HOME.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

bin/find-spark-home

@@ -33,9 +33,9 @@ elif [ ! -f "$FIND_SPARK_HOME_PYTHON_SCRIPT" ]; then
   export SPARK_HOME="$(cd "$(dirname "$0")"/..; pwd)"
 else
   # We are pip installed, use the Python script to resolve a reasonable SPARK_HOME
-  # Default to standard python interpreter unless told otherwise
+  # Default to standard python3 interpreter unless told otherwise
   if [[ -z "$PYSPARK_DRIVER_PYTHON" ]]; then
-     PYSPARK_DRIVER_PYTHON="${PYSPARK_PYTHON:-"python"}"
+     PYSPARK_DRIVER_PYTHON="${PYSPARK_PYTHON:-"python3"}"
   fi
   export SPARK_HOME=$($PYSPARK_DRIVER_PYTHON "$FIND_SPARK_HOME_PYTHON_SCRIPT")
 fi

bin/find-spark-home.cmd

@@ -20,8 +20,8 @@ rem
 rem Path to Python script finding SPARK_HOME
 set FIND_SPARK_HOME_PYTHON_SCRIPT=%~dp0find_spark_home.py
 
-rem Default to standard python interpreter unless told otherwise
-set PYTHON_RUNNER=python
+rem Default to standard python3 interpreter unless told otherwise
+set PYTHON_RUNNER=python3
 rem If PYSPARK_DRIVER_PYTHON is set, it overwrites the python version
 if not "x%PYSPARK_DRIVER_PYTHON%"=="x" (
   set PYTHON_RUNNER=%PYSPARK_DRIVER_PYTHON%

bin/pyspark

@@ -37,9 +37,9 @@ if [[ -n "$IPYTHON" || -n "$IPYTHON_OPTS" ]]; then
   exit 1
 fi
 
-# Default to standard python interpreter unless told otherwise
+# Default to standard python3 interpreter unless told otherwise
 if [[ -z "$PYSPARK_PYTHON" ]]; then
-  PYSPARK_PYTHON=python
+  PYSPARK_PYTHON=python3
 fi
 if [[ -z "$PYSPARK_DRIVER_PYTHON" ]]; then
   PYSPARK_DRIVER_PYTHON=$PYSPARK_PYTHON