[SPARK-3867][PySpark] ./python/run-tests failed when it run with Python 2.6 and unittest2 is not installed

./python/run-tests search a Python 2.6 executable on PATH and use it if available.
When using Python 2.6, it is going to import unittest2 module which is not a standard library in Python 2.6, so it fails with ImportError.

Author: cocoatomo <cocoatomo77@gmail.com>

Closes #2759 from cocoatomo/issues/3867-unittest2-import-error and squashes the following commits:

f068eb5 [cocoatomo] [SPARK-3867] ./python/run-tests failed when it run with Python 2.6 and unittest2 is not installed
[SPARK-3909][PySpark][Doc] A corrupted format in Sphinx documents and building warnings

Sphinx documents contains a corrupted ReST format and have some warnings.

The purpose of this issue is same as https://issues.apache.org/jira/browse/SPARK-3773.

commit: 0e8203f4fb721158fb27897680da476174d24c4b

output
```
$ cd ./python/docs
$ make clean html
rm -rf _build/*
sphinx-build -b html -d _build/doctrees   . _build/html
Making output directory...
Running Sphinx v1.2.3
loading pickled environment... not yet created
building [html]: targets for 4 source files that are out of date
updating environment: 4 added, 0 changed, 0 removed
reading sources... [100%] pyspark.sql
/Users/<user>/MyRepos/Scala/spark/python/pyspark/mllib/feature.py:docstring of pyspark.mllib.feature.Word2VecModel.findSynonyms:4: WARNING: Field list ends without a blank line; unexpected unindent.
/Users/<user>/MyRepos/Scala/spark/python/pyspark/mllib/feature.py:docstring of pyspark.mllib.feature.Word2VecModel.transform:3: WARNING: Field list ends without a blank line; unexpected unindent.
/Users/<user>/MyRepos/Scala/spark/python/pyspark/sql.py:docstring of pyspark.sql:4: WARNING: Bullet list ends without a blank line; unexpected unindent.
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] pyspark.sql
writing additional files... (12 module code pages) _modules/index search
copying static files... WARNING: html_static_path entry u'/Users/<user>/MyRepos/Scala/spark/python/docs/_static' does not exist
done
copying extra files... done
dumping search index... done
dumping object inventory... done
build succeeded, 4 warnings.

Build finished. The HTML pages are in _build/html.
```

Author: cocoatomo <cocoatomo77@gmail.com>

Closes #2766 from cocoatomo/issues/3909-sphinx-build-warnings and squashes the following commits:

2c7faa8 [cocoatomo] [SPARK-3909][PySpark][Doc] A corrupted format in Sphinx documents and building warnings