Externals Update, master branch (2020.10.29.)

Updated all projects to atlasexternals-2.0.84.

The only update compared to atlasexternals-2.0.83 (atlasexternals@2.0.83...2.0.84) is the addition of a Findzipp.cmake module, and it making use of it while setting up flake8. This is necessary to fix the python warnings that showed up with the Athena,master,r2020-10-28T2101 nightly.

Note that for this update full-build is really not needed. (The CI is not going to be even able to test if the fix works as it should or not...)

Pinging @fwinkl.