### What changes were proposed in this pull request?
This PR aims to test PySpark with Python 3.8 in Github Actions. In the script side, it is already ready:
4ad9bfd53b/python/run-tests.py (L161)
This PR includes small related fixes together:
1. Install Python 3.8
2. Only install one Python implementation instead of installing many for SQL and Yarn test cases because they need one Python executable in their test cases that is higher than Python 2.
3. Do not install Python 2 which is not needed anymore after we dropped Python 2 at SPARK-32138
4. Remove a comment about installing PyPy3 on Jenkins - SPARK-32278. It is already installed.
### Why are the changes needed?
Currently, only PyPy3 and Python 3.6 are being tested with PySpark in Github Actions. We should test the latest version of Python as well because some optimizations can be only enabled with Python 3.8+. See also https://github.com/apache/spark/pull/29114
### Does this PR introduce _any_ user-facing change?
No, dev-only.
### How was this patch tested?
Was not tested. Github Actions build in this PR will test it out.
Closes#29116 from HyukjinKwon/test-python3.8-togehter.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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