fix pythonpath by using generator expression
With multi-configuration/crossplatform toolchains, the output path of a target varies from the selected configuration (most famous: Debug/Release on MSVC/xcode).
Therefore, the module's output path is not the bin dir, but mostly in something like ${CMAKE_CURRENT_BINARY_DIR}/Debug or ${CMAKE_CURRENT_BINARY_DIR}/Release.
This MR fixes the path, so that pythonpath is always set to the correct path where the module was built into.
