Un-pin alembic-verify and remove sqlalchemy-diff dependency

[jantman]

See 359f779 and 342888f and gianchub/alembic-verify#15

alembic-verify previously depended on sqlalchemy-diff and we had to pin to an old version to get it to work. alembic-verify now has new versions out (1.0.2 seems to be the latest as of when this issue was opened, but we should check for the current version at time of implementation) which no longer depend on sqlalchemy-diff.

Remove the sqlalchemy-diff dependency completely, update alembic-verify to the latest version, and make whatever code changes are required for the significant changes (e.g. fixture name changes) in the 1.x versions.