buildbot.test.integration.test_upgrade.UpgradeTestEmpty.test_emptydb_modelmatches fails after alembic upgrade

[p12tic]

#7441 upgraded Alembic which enabled type comparisons by default (see https://alembic.sqlalchemy.org/en/latest/changelog.html#change-1.12.0). This broke buildbot.test.integration.test_upgrade.UpgradeTestEmpty.test_emptydb_modelmatches test as seen in https://buildbot.buildbot.net/#/builders/89/builds/8025/steps/11:

[FAIL]
Traceback (most recent call last):
File "/buildbot/buildbot-job/build/master/buildbot/test/integration/test_upgrade.py", line 239, in test_emptydb_modelmatches
yield self.assertModelMatches()
File "/tmp/bbvenv/lib/python3.9/site-packages/twisted/internet/defer.py", line 2000, in _inlineCallbacks
result = context.run(gen.send, result)
File "/buildbot/buildbot-job/build/master/buildbot/test/integration/test_upgrade.py", line 198, in assertModelMatches
self.fail("\n" + pprint.pformat(diff))
twisted.trial.unittest.FailTest:
[[('modify_type',
None,
'logchunks',
'content',
{'existing_comment': None,
'existing_nullable': True,
'existing_server_default': False},
MEDIUMBLOB(),
LargeBinary(length=65536))]]
buildbot.test.integration.test_upgrade.UpgradeTestEmpty.test_emptydb_modelmatches

This seems like a bug in some past migration and should be addressed.