release: Quibble 1.5.6

• Remove MW_QUIBBLE_CI environment variable
• Pass --innodb-print-all-deadlocks to MariaDB
• Replace deprecated setuptools license_file with license_files
• Bump setuptools-git-versioning to at least 1.8.0
• Remove --run=all (default is to run all stages)
• Add --change to retrieve a single change from Gerrit to test against
• Document how to test Quibble changes
• Remove parsoid from utils/ci-fullrun-extensions.sh
• Remove files remaining after removal of Docker support
• Remove Sphinx setuptools integration (build_sphinx)
• Refresh Sphinx conf.py

Bug: T342088
Change-Id: Idc5af338564ab9b52e615c18a44376bed295e11f