feat: support setting the pythonpath in configuration YAMLs

[rpoisel]

Description

The pythonpath key allows specifying additional directories to be added to the Python search path (sys.path). This is particularly useful for including directories that contain Python modules and packages which are not installed in the standard library or not in the current directory.

Relative paths are resolved relative to the configuration file's location.

Specifying this option enables the environment to locate and import modules that are not in the default Python path.

Checklist

• [x] Documentation for the feature
• [x] Tests for the feature
• [x] The arguments and description in doc/configuration.rst have been updated
• [ ] Add a section on how to use the feature to doc/usage.rst
• [ ] Add a section on how to use the feature to doc/development.rst
• [ ] PR has been tested
• [x] Man pages have been regenerated