### 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#29246 from dongjoon-hyun/SPARK-FIND-SPARK-HOME.
Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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