## What changes were proposed in this pull request?
Not a big deal but this PR adds `sphinx` into `dev/requirements.txt` since we found it needed - https://github.com/apache/spark-website/pull/122#discussion_r200896018
## How was this patch tested?
manually:
```
pip install -r requirements.txt
```
Author: hyukjinkwon <gurwls223@apache.org>
Closes#21735 from HyukjinKwon/minor-dev.
## What changes were proposed in this pull request?
Fix instalation of mllib and ml sub components, and more eagerly cleanup cache files during test script & make-distribution.
## How was this patch tested?
Updated sanity test script to import mllib and ml sub-components.
Author: Holden Karau <holden@us.ibm.com>
Closes#16465 from holdenk/SPARK-19064-fix-pip-install-sub-components.
Minor since so few people use them, but it would probably be good to have a requirements file for our python release tools for easier setup (also version pinning).
cc JoshRosen who looked at the original JIRA.
Author: Holden Karau <holden@us.ibm.com>
Closes#10871 from holdenk/SPARK-10498-add-requirements-file-for-dev-python-tools.
