fix: escape paths with spaces in pybind11-config

[MarkusBauer]

Description

tl;dr: pybind11 --includes returns invalid arguments when the project's directory contains spaces.

When you build an extension manually, the docs suggest this command: c++ ... $(python3 -m pybind11 --includes) ....
However, this command fails when the project's directory contains a space in its path.
The command would then expand to c++ ... -I/home/user/a b/c -I/python ..., where /home/user/a is a in invalid include directory, and b/c is an invalid compiler argument.

Even if no manual build is desired this bug might affect users: for example, we're currently building clang-tidy in our project, which requires all include paths.

To solve this issue, this PR escapes include paths of pybind11 --includes before printing. Paths without spaces/special characters are printed as-is, while paths with spaces are wrapped in ''. Thus, the final command line of custom builds will be valid even if spaces occur: c++ ... '-I/home/user/a b/c' -I/python ...

Suggested changelog entry:

* Fix paths with spaces, including on Windows

[rwgk]

Thanks!

Waiting for @henryiii to help with a 2nd set of eyes.

[henryiii]

I'd forgotten about this. Unless mslex modernizes a bit (it currently uses setup.py with deprecated arguments, like test_requires), I think I'd like to just have the custom form (like meson). I think we can go a bit simpler, though, since double quotes are not valid in a Windows path, only single quotes are. I'll update this.

[rwgk]

... alright ...

I cannot honestly write "looks good to me" here, but it seem beyond anyone's capacity (definitely my own) to do something actually good here :-)

I'm holding out hope that some future Python version will come with a platform-aware rock-solid quote() implementation out of the box.

[QuLogic]

FWIW, I just tried this on Windows, and everything is quoted enough to work properly, and cl no longer warns about non-existent paths.

[henryiii]

Released.