So far, I have not observed any failures after removing the offending lines ("sqlalchemy >= 0.6, <= 0.7.10" and "sqlalchemy-migrate==0.7.2") from /usr/lib/python2.7/dist-packages/buildbot-0.8.12.egg-info/requires.txt
Tested with buildbot 0.8.12-3.2 on stretch, with python-sqlalchemy 1.0.15+ds1-1 and python-migrate 0.10.0-7. Also installed "python-mock" and ran the Buildbot tests with trial, though I am not sure how much of the migration code gets exercised in the tests. This commit to upstream seems to indicate that the newer versions of sqlalchemy and sqlalchemy-migrate included in Debian should be compatible with each other: https://github.com/buildbot/buildbot/commit/222361a0e061291c9ba7fd7e6a660c7356ecd218 "Accept sqlalchemy-migrate versions 0.8 and up as compatible with sqlalchemy version 0.8 and up."