• removing unused imports as suggested by lgtm and pyflakes

• mark some specific files as catalog-like in lgtm.yml

CC: @tscrim @jm58660 @vinklein

Component: refactoring

Author: Frédéric Chapoton

Branch/Commit: e2c8680

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/29068