Update to LCG_100_ATLAS_3 for distro python package
This is a follow-up MR !43271 (merged) with an upgrade to LCG_100_ATLAS_3.
As discussed in https://its.cern.ch/jira/browse/ATLASRECTS-6271, the platform.linux_distribution() call is not supported anymore in python 3.8 and is now replaced by distro.linux_distribution() in the PyJobTransform package.
Keeping this MR as in draft status until MR !43668 (merged) is merged.
For the RC: since this MR updates the LCG layer, perhaps clean all the CI build nodes after the MR is accepted.
Edited by Johannes Elmsheuser