import_step: do not assign labels to Compound.for_construction

[mbugert]

This is a proposed fix for #662.

After importing a STEP file, the for_construction attribute of Compounds contain an OCP.TDF.TDF_Label. This will make Shape.__deepcopy__ fail because the label is not pickleable.
I removed the assignment of TDF_Labels to for_construction in the STEP import, and pass the label as a function argument to build_assembly instead.
I tried importing a few STEP files and it appears to work. WDYT?

[gumyr]

Thanks! I had originally expected to do more with the TDF_Label stored temporarily in the for_construction attribute but as you've shown it isn't really required.