## What changes were proposed in this pull request?
This PR proposes to add `SPHINXPYTHON` environment variable to control the Python version used by Sphinx.
The motivation of this environment variable is, it seems not properly rendering some signatures in the Python documentation when Python 2 is used by Sphinx. See the JIRA's case. It should be encouraged to use Python 3, but looks we will probably live with this problem for a long while in any event.
For the default case of `make html`, it keeps previous behaviour and use `SPHINXBUILD` as it was. If `SPHINXPYTHON` is set, then it forces Sphinx to use the specific Python version.
```
$ SPHINXPYTHON=python3 make html
python3 -msphinx -b html -d _build/doctrees . _build/html
Running Sphinx v1.7.5
...
```
1. if `SPHINXPYTHON` is set, use Python. If `SPHINXBUILD` is set, use sphinx-build.
2. If both are set, `SPHINXBUILD` has a higher priority over `SPHINXPYTHON`
3. By default, `SPHINXBUILD` is used as 'sphinx-build'.
Probably, we can somehow work around this via explicitly setting `SPHINXBUILD` but `sphinx-build` can't be easily distinguished since it (at least in my environment and up to my knowledge) doesn't replace `sphinx-build` when newer Sphinx is installed in different Python version. It confuses and doesn't warn for its Python version.
## How was this patch tested?
Manually tested:
**`python` (Python 2.7) in the path with Sphinx:**
```
$ make html
sphinx-build -b html -d _build/doctrees . _build/html
Running Sphinx v1.7.5
...
```
**`python` (Python 2.7) in the path without Sphinx:**
```
$ make html
Makefile:8: *** The 'sphinx-build' command was not found. Make sure you have Sphinx installed, then set the SPHINXBUILD environment variable to point to the full path of the 'sphinx-build' executable. Alternatively you can add the directory with the executable to your PATH. If you don't have Sphinx installed, grab it from http://sphinx-doc.org/. Stop.
```
**`SPHINXPYTHON` set `python` (Python 2.7) with Sphinx:**
```
$ SPHINXPYTHON=python make html
Makefile:35: *** Note that Python 3 is required to generate PySpark documentation correctly for now. Current Python executable was less than Python 3. See SPARK-24530. To force Sphinx to use a specific Python executable, please set SPHINXPYTHON to point to the Python 3 executable.. Stop.
```
**`SPHINXPYTHON` set `python` (Python 2.7) without Sphinx:**
```
$ SPHINXPYTHON=python make html
Makefile:35: *** Note that Python 3 is required to generate PySpark documentation correctly for now. Current Python executable was less than Python 3. See SPARK-24530. To force Sphinx to use a specific Python executable, please set SPHINXPYTHON to point to the Python 3 executable.. Stop.
```
**`SPHINXPYTHON` set `python3` with Sphinx:**
```
$ SPHINXPYTHON=python3 make html
python3 -msphinx -b html -d _build/doctrees . _build/html
Running Sphinx v1.7.5
...
```
**`SPHINXPYTHON` set `python3` without Sphinx:**
```
$ SPHINXPYTHON=python3 make html
Makefile:39: *** Python executable 'python3' did not have Sphinx installed. Make sure you have Sphinx installed, then set the SPHINXPYTHON environment variable to point to the Python executable having Sphinx installed. If you don't have Sphinx installed, grab it from http://sphinx-doc.org/. Stop.
```
**`SPHINXBUILD` set:**
```
$ SPHINXBUILD=sphinx-build make html
sphinx-build -b html -d _build/doctrees . _build/html
Running Sphinx v1.7.5
...
```
**Both `SPHINXPYTHON` and `SPHINXBUILD` are set:**
```
$ SPHINXBUILD=sphinx-build SPHINXPYTHON=python make html
sphinx-build -b html -d _build/doctrees . _build/html
Running Sphinx v1.7.5
...
```
Author: hyukjinkwon <gurwls223@apache.org>
Closes#21659 from HyukjinKwon/SPARK-24530.
1f94bf492c