Add flake8_builtins and flake8_bugbear to top_packages of 104d_ATLAS_15 and 104d_ATLAS_16

To run the testing, add a comment with:

Bot test PACKAGE[ PACKAGE2 ...]|all in TOOLCHAIN|all [(only|also) for ARCH-LABEL-COMPILER-BUILDTYPE] please!

later followed by

Bot repeat please!

To ensure patches required for existing LCG branches are not lost, please add the respective labels: alsoTargeting:LCG_102-patches, alsoTargeting:master, etc.

Closes #104