Sphinx fails to create warning log file if directory does not exist

[arwedus]

Describe the bug

In a project with the following directory structure:

|- build
|- source
|  |- conf.py
|  `- index.rst
|- Makefile

and the following command to build:

 sphinx-build -M html source build -w build/sphinx_build_warnings.log 

I get the following error if the "build" directory does not exist yet:

sphinx-build: error: cannot open warning file 'build/sphinx_build_warnings.log': [Errno 2] No such file or directory: 'build/sphinx_build_warnings.log'

How to Reproduce

$ sphinx-build -M html source build -w build/sphinx_build_warnings.log 

Expected behavior

I would expect sphinx-build to create the relative path to the warning logfile for me instead of failing, because this is how it handles the build output path as well.

Your project

should not be necessary

Screenshots

No response

OS

Linux Ubuntu 20.04

Python version

3.8.9

Sphinx version

4.3.1

Sphinx extensions

No response

Extra tools

No response

Additional context

No response