Reduce pystring usages

[meimchu]

First step of pystring removal by improving StringUtils (#2256)
StringUtils have been updated to include some pystring functionalities. Focused on replacing:

• pystring::split()
• pystring::strip()
• pystring::replace()
• pystring::mul()
• pystring::startswith()
• pystring::endswith()

These usages have also been replaced throughout the modules that use these as well. Only pystring::os usages should be left now.

Additional changes of note:

• Temporarily changed ci_workflow.yml. Additional explanation below.
• Added an additional include path for OpenFX build.

@KevinJW please take a look when you have a moment! Thank you.

[meimchu]

I have temporarily removed a couple of combinations of our CICD Linux tests here to at least allow more tests to run. Since one tests from the OS category can stop the other jobs from running.

There is an on-going discussions in Slack regarding some Linux combinations and the recent aswf/ci-ocio:2023 docker image update. That is not the focus of this PR so I have temporarily removed it as a distraction.

I will not merge this request without a conclusion is reached in that regards.

The initial message from Jean-Francois Panisset:

See my comments in #wg-ci, it does seem like the update of the aswf/ci-ocio:2023 image from 2023.2 to 2023.3 has "changed something", and now the self build of OpenEXR 3.4 (the version from 2025) is breaking since that makes use of C++ 20 features.

Quick workaround would be to pin this build to use the aswf/ci-ocio:2023.2 image which should restore the previous behavior, but more generally it would be interesting to figure out "what does building latest OCIO with VFX 2025 OpenEXR in a VFX 2023 environment" really mean, and is that something we need / want to support.