Remove cachetools after Python 3 migration

!3003 (merged) and Moore!791 (merged) added a few usages of cachetools to the configuration. LBCORE-2006 added cachetools to LCG_97a and LCG_97apython3, but we don't expect it to be available in LCG_100 because we will migrate to Python 3 then and can use functools directly.

We should remove this and other instances of cachetools once the stack has migrated to Python 3 and there are no Python 2 slots against the master branch in the nightlies.

/cc @bcouturi

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