Oh, I overlooked that it is about LSP (and not Robot Code).
With LSP you can avoid typing the full path by using some variables.
So you can add the Pythonpath like that to the settings:
"robot.pythonpath": [
"${workspaceFolder}", "${workspaceFolder}/lib", "${workspaceFolder}/resources", "${workspaceFolder}/keywords"
]
${workspaceFolder} means here your project root folder.
It will be resolved to the absolute path